The average age at which OECD women give birth for the first time climbed from 28.6 in 2000 to 30.9 in 2022, a shift that occurred almost entirely during Millennials’ and older Gen Z’s prime childbearing years. Generation Z is now moving through that same life stage, and how it resolves the same pressures, housing costs, labor market entry, and shifting attitudes toward parenthood, will do more to determine national population trajectories over the next thirty years than any policy currently on the table.
Gen Z’s Demographic Starting Point
Generation Z, born roughly 1997 to 2012, is estimated at close to 2 billion people globally, a figure comparable to Millennials and Generation Alpha, since global annual births have remained relatively steady across all three generational windows even as fertility per woman has declined. Gen Z’s geographic distribution mirrors the broader global divide already visible in younger cohorts: a large share concentrated in South Asia and Sub-Saharan Africa, with a shrinking share in East Asia and Europe, where sub-replacement fertility has persisted for over a decade longer than in most of the rest of the world.
By 2024, the oldest members of Generation Z had reached their late twenties, placing them squarely in the years when family formation decisions are typically made, while the youngest members were still in early adolescence, meaning any assessment of Gen Z’s eventual demographic impact necessarily blends observed early data with projection for a cohort that has not finished its childbearing years.
Gen Z and the Transition Into Adulthood
Several measurable indicators show Generation Z, following a pattern already established by Millennials, taking longer to reach conventional adulthood milestones than earlier generations did at the same age. In the United States, 20.4 percent of adults aged 25 to 29 lived with their parents in 2025, nearly six percentage points above the rate at the start of the century, while 12.7 percent of 30-to-34-year-olds did the same, nearly double the 7.1 percent recorded in 2000.
In the United Kingdom, co-residence among 25-to-34-year-olds rose from 13 percent in 2006 to 18 percent in 2024, representing roughly 450,000 more young adults living with parents than if the 2006 rate had held steady. Across the OECD as a whole, around half of all 20-to-29-year-olds still lived with their parents as of the most recent available data, with levels considerably higher in Southern Europe than in Nordic countries.
Delayed Family Formation and Fertility
The clearest demographic signal from this delayed transition is a later average age at first birth. OECD-wide, that average rose to 30.9 in 2022 from 28.6 in 2000. Individual countries show similar or steeper increases: mean age at childbirth in Canada rose from 28.8 in 2000 to 31.6 in 2022, while in the United States it rose from 27.4 to 29.5 over the same period.
The OECD’s own analysis attributes much of this shift to housing costs, describing a measurable negative relationship between rising housing costs and total fertility rates across member countries, compounded by economic insecurity linked to the 2008 financial crisis, the COVID-19 pandemic, and the cost-of-living pressures that followed.
It would be a mistake to read delayed childbearing as certain evidence that Generation Z will have fewer children than intended. Delay and reduction are related but distinct outcomes: some delayed births still occur later, while others do not occur at all, a distinction reflected in rising childlessness rates.
Among women born in 1975, a cohort now largely finished with childbearing, close to one in four in Italy and Spain remained permanently childless, and the figure reached 28 percent in Japan. Whether Generation Z, arriving at these same decisions a decade or two later and under different economic conditions, will show a similar or different childlessness pattern remains genuinely unknown.
Delayed Adulthood Indicators
| Indicator | Earlier Period | Recent Period |
|---|---|---|
| OECD average age at first birth | 28.6 (2000) | 30.9 (2022) |
| U.S. mean age at childbirth | 27.4 (2000) | 29.5 (2022) |
| Canada mean age at childbirth | 28.8 (2000) | 31.6 (2022) |
| U.S. adults 25-29 living with parents | approx. 14.5% (2000) | 20.4% (2025) |
| U.S. adults 30-34 living with parents | 7.1% (2000) | 12.7% (2025) |
| UK co-residence, ages 25-34 | 13% (2006) | 18% (2024) |
| OECD average fertility rate | approx. 1.6-1.7 (early 2000s) | 1.5 (2022) |
Gen Z and Migration
Migration is likely to play a larger role in shaping Generation Z’s demographic footprint than it did for earlier generations, for a straightforward reason: the OECD and other analyses increasingly point to immigration as a policy lever for offsetting shrinking working-age populations as fertility remains below replacement across most member countries.
Generation Z members in destination countries with significant immigration are entering labor markets and communities that are becoming more demographically mixed than those their parents entered, while Generation Z members in origin countries with high youth populations and limited domestic job creation, common across parts of Sub-Saharan Africa and South Asia, face continued pressure toward outward migration in search of economic opportunity.
Gen Z and Labour Force Replacement
Generation Z is entering labor markets in most high-income countries at a moment when the working-age population is growing more slowly, or in several cases already shrinking, than it has in decades. This dynamic is a direct consequence of the smaller birth cohorts that followed the Baby Boom, combined with the sustained sub-replacement fertility that has defined most of Generation Z’s own birth years.
In countries including Japan, South Korea, Italy, and China, Generation Z represents a comparatively small replacement cohort relative to the far larger Baby Boomer and Generation X populations now moving into retirement, a mismatch that underlies much of the current policy discussion around raising retirement ages, expanding immigration, and increasing labor force participation among women and older workers.
Countries Where Gen Z Could Be Demographically Transformative
In countries with younger, still-growing populations, largely across Sub-Saharan Africa, Gen Z’s sheer size relative to the existing working-age population creates the conditions for what demographers call a demographic dividend, a period of potentially accelerated economic growth driven by a favorable ratio of working-age people to dependents.
Realizing that dividend depends heavily on whether education systems and labor markets can absorb a large youth cohort with productive employment, a challenge several countries in the region have struggled to meet, given that many high-fertility countries also face constraints on school capacity, infrastructure, and job creation.
Countries Where Gen Z Is Entering an Ageing Population
In much of East Asia and Europe, Generation Z is entering societies already well advanced in population aging, with old-age dependency ratios projected to keep climbing through the 2030s and 2040s. South Korea and Japan represent the most advanced cases, where Gen Z workers already face the prospect of supporting proportionally more retirees over their careers than any previous generation of workers has had to. This dynamic places pressure on pension systems, healthcare financing, and immigration policy in ways that will shape Generation Z’s economic and political environment throughout their working lives, independent of their own individual choices about family formation.
What Could Change by 2050
By 2050, the oldest members of Generation Z will be in their early fifties, meaning most of the cohort’s major life-course demographic decisions, childbearing, migration, labor force participation, will have already been made or substantially resolved. UN population projections suggest global population growth will have slowed considerably by then, with a growing number of countries beyond the current 63 that have already peaked adding to that list.
Whether Generation Z’s own fertility choices accelerate or moderate that trajectory in specific countries remains one of the more consequential open questions in global demography, given how much weight a single generation’s family formation decisions carry for population trends that will not become clear for another two to three decades.
Key Findings
- OECD average age at first birth rose from 28.6 in 2000 to 30.9 in 2022, with similar or steeper increases recorded in the U.S. and Canada, largely coinciding with Millennials’ and older Gen Z’s prime childbearing years.
- U.S. co-residence with parents among 25-to-29-year-olds reached 20.4 percent in 2025, up nearly six percentage points from 2000, while the 30-to-34 rate nearly doubled over the same period.
- The OECD’s fertility rate averaged 1.5 in 2022, with South Korea’s rate falling to an estimated 0.7 in 2023, the lowest of any OECD member and among the lowest ever recorded for any country.
- Among women born in 1975, close to one in four remained permanently childless in Italy and Spain, and 28 percent did in Japan, indicating that delayed childbearing among younger cohorts can translate into higher lifetime childlessness rather than simply births occurring later.
- Generation Z is entering labor markets in Japan, South Korea, Italy, and China as a comparatively small replacement cohort relative to much larger retiring Baby Boomer and Generation X populations, intensifying pressure on pension and healthcare systems.
Conclusion
Generation Z’s demographic legacy will not be determined by its size at birth, which is already known, but by decisions its members have not yet finished making: when and whether to have children, where to live and work, and how to navigate labor markets shaped by aging populations in some countries and youth-driven growth in others.
The clearest evidence so far, later average age at first birth, rising co-residence with parents, and mixed signals on eventual family size, points toward a generation delaying rather than abandoning traditional adulthood milestones, but delay itself is already reshaping national population trajectories in measurable ways.
Frequently Asked Questions
1. Will Gen Z have fewer children than previous generations?
It is too early to say definitively, since much of the cohort has not completed its childbearing years. Delayed childbearing, already evident in rising average age at first birth, does not automatically mean fewer total children, though rising childlessness rates among the preceding Millennial cohort suggest delay and reduction are often linked.
2. How large is Gen Z?
Generation Z is estimated at roughly 2 billion people worldwide, comparable in size to Millennials and Generation Alpha, reflecting relatively stable global annual birth counts across all three generational windows.
3. Will Gen Z increase migration?
Migration is likely to play a significant role in Generation Z’s demographic story, both as a policy tool used by aging, low-fertility countries to offset shrinking working-age populations, and as a life choice made by Gen Z members in countries with high youth populations and limited domestic economic opportunity.
4. How will Gen Z affect population aging?
In low-fertility countries, Generation Z represents a comparatively small replacement cohort relative to the much larger Baby Boomer and Generation X populations now retiring, intensifying old-age dependency pressures. In high-fertility countries, Gen Z’s large relative size creates potential for a demographic dividend instead.
5. Why is the average age at first birth rising?
OECD research links the trend to a combination of factors, including rising housing costs, extended time in education, economic insecurity following the 2008 financial crisis and the COVID-19 pandemic, and changing attitudes that increasingly find meaning in life outside of parenthood.
6. Are more young adults living with their parents than in the past?
Yes, in most countries examined. In the U.S., co-residence rates for both 25-to-29 and 30-to-34-year-olds have risen substantially since 2000. In the UK, co-residence for 25-to-34-year-olds rose from 13 percent in 2006 to 18 percent in 2024.
7. Which countries have the lowest fertility rates among current OECD members?
South Korea recorded an estimated fertility rate of 0.7 in 2023, the lowest in the OECD, followed by other countries in East Asia and Southern Europe including Italy and Spain, both around 1.2 children per woman.
8. Does delayed adulthood mean Gen Z is choosing not to have children?
Not necessarily. OECD research points to structural economic factors, particularly housing costs and labor market insecurity, as significant drivers of delay, alongside a genuine cultural shift toward finding meaning outside of parenthood for some individuals.
9. Will Gen Z reverse or continue declining fertility trends?
This remains genuinely uncertain and will only become clear as more of the cohort completes its childbearing years over the next one to two decades. Some countries, including South Korea, have shown tentative recent upticks in births, though it is too early to call these confirmed trend reversals.
10. How will Gen Z affect the labor force in aging countries?
In countries like Japan, South Korea, and Italy, Generation Z is a smaller replacement cohort than the retiring generations before it, a mismatch driving policy discussions around raising retirement ages, expanding immigration, and increasing labor force participation among women and older workers.
Sources
- OECD, “Society at a Glance 2024: A spotlight on fertility trends,” OECD Publishing, 2024
- OECD, Affordable Housing Database, Indicator HM1.4, “Living arrangements by age groups,” 2024
- OECD, Society at a Glance 2024 Country Notes, United States and Canada
- Institute for Fiscal Studies, “Hotel of Mum and Dad? Co-residence with parents among those aged 25-34,” UK, 2024
- Yahoo Finance/Redfin analysis, U.S. co-residence data, 2025
- United Nations, World Population Prospects 2024
