South Korea’s fertility rate stood near 1.5 births per woman when the oldest Millennials were born in the early 1980s. By the time Generation Z’s oldest members were born in the late 1990s, it had fallen below 1.5 and kept falling, reaching an estimated 0.7 by 2023, the lowest rate ever recorded by an OECD member. That single trajectory, compressed into roughly two decades, captures the core difference between these two generations better than any comparison of their attitudes or habits: Millennials came of age as fertility decline was still unfolding in most of the world, while Gen Z is coming of age after it has already reached historic lows in a growing number of countries.
Defining the Two Cohorts
Pew Research Center defines Millennials as people born between 1981 and 1996, and treats the boundary with Generation Z, generally placed at 1997, as an analytical convention rather than a hard demographic line. Pew has been explicit that generational boundaries are useful primarily for grouping people into similarly sized analytical units for research purposes, not because a specific birth year produces some sudden, universal shift in outlook.
With that caveat in place, Millennials are currently in their late twenties through early forties, occupying prime working and family-formation years, while Generation Z spans early adolescence through the early thirties, meaning the two generations currently overlap heavily in workplaces and, increasingly, in parenthood.
The Population World Millennials Entered
Millennials were born into a world still experiencing relatively high global population growth. Global fertility in the early 1980s sat well above 3 births per woman, and global population growth rates, while already declining from their 1960s peak, remained considerably higher than they are today.
Most large economies, including the United States, still had fertility rates near or above replacement level for at least part of the Millennial birth window. China’s one-child policy, in effect for the middle portion of the Millennial cohort’s birth years, was a significant exception, already producing a much lower Chinese fertility rate than global or Millennial-era Western norms.
The Population World Gen Z Is Entering
By the time Generation Z began being born in 1997, global fertility had already fallen to roughly 2.7 births per woman, and it continued falling steadily throughout Gen Z’s birth years, reaching approximately 2.2 by 2024. More significantly, the number of countries with sub-replacement fertility expanded considerably during this same period, moving from a minority of mostly wealthy countries in the 1990s to encompassing more than half of all countries and areas worldwide by the mid-2020s. Gen Z is also the first generation whose entire birth window overlaps with a documented, sustained increase in the number of countries reaching outright population peak, a milestone that had occurred in relatively few countries before the 2000s.
Fertility and Growth Comparison
| Indicator | Millennials’ Birth Era (approx. 1981-1996) | Gen Z’s Birth Era (approx. 1997-2012) |
|---|---|---|
| Global fertility rate | approx. 3.3-3.7 (early) to approx. 2.9 (late) | approx. 2.7 (early) to approx. 2.5 (late) |
| Countries below replacement fertility | Minority, mostly high-income | Growing steadily toward roughly half of all countries |
| South Korea fertility rate | approx. 1.5-2.8 (declining across period) | approx. 0.8-1.5 (declining sharply across period) |
| Global population growth rate | approx. 1.7-1.8% per year | approx. 1.2-1.3% per year |
Family Formation and Delayed Adulthood
Millennials were the first generation to show the now-familiar pattern of delayed marriage, delayed homeownership, and delayed childbearing at scale in most high-income countries, a pattern OECD research links substantially to rising housing costs and labor market insecurity following the 2008 financial crisis, an event that hit many Millennials directly as they were entering the workforce. Generation Z is now showing the same pattern, but starting from an already-delayed baseline and facing a second major economic disruption, the COVID-19 pandemic, during its own transition to adulthood.
U.S. data illustrates the compounding effect. Co-residence with parents among 25-to-29-year-olds reached 20.4 percent in 2025, a rate incorporating both older Gen Z and younger Millennials, up nearly six percentage points from year-2000 levels that primarily reflected the generation before Millennials.
The OECD’s average age at first birth rose from 28.6 in 2000, a period dominated by older Millennial parents, to 30.9 by 2022, a period increasingly including Gen Z parents, suggesting the delay identified in Millennials has not reversed for Gen Z but has continued or deepened.
Population Geography
Both generations show similar broad geographic patterns, concentrated more heavily in South Asia and Sub-Saharan Africa than in East Asia or Europe, but the concentration has intensified for Gen Z. Sub-Saharan Africa’s share of global births rose across the two generational windows as the region’s fertility decline proceeded more slowly than elsewhere, meaning a larger proportion of Generation Z, relative to Millennials, was born in the region.
Conversely, East Asia’s and Europe’s shares of global births continued shrinking across the same period, meaning Millennials include a comparatively larger share of the generation born in now-low-fertility, high-income countries than Generation Z does.
Fertility and Future Family Size
Millennials, now largely past their peak childbearing years in most high-income countries, offer the clearer picture: rising childlessness, with close to one in four women born in the mid-1970s, an age group bridging late Generation X and the oldest Millennials, remaining permanently childless in Italy and Spain, and 28 percent in Japan.
Generation Z’s eventual family size remains substantially unresolved, since much of the cohort has not yet reached, or has only recently reached, prime childbearing age. Whatever fertility patterns Generation Z ultimately shows will be shaped by economic and housing conditions specific to the 2025-2040 period, which may or may not resemble the conditions Millennials faced in the 2005-2020 period.
Which Generation Faces the Greater Demographic Challenge
This question resists a single answer, because “greater challenge” depends on which demographic dimension is being measured. By the measure of entering adulthood during major economic disruption, both generations qualify: Millennials with the 2008 financial crisis, Generation Z with the COVID-19 pandemic and its aftermath. By the measure of inherited population aging,
Generation Z faces a more advanced version of the same challenge Millennials encountered, since the countries that were beginning to age noticeably during Millennials’ young adulthood, Japan and much of Europe, have aged considerably further by the time Generation Z reaches the same life stage. By the measure of raw generational size, both cohorts land in a similar range globally, near 2 billion people, meaning neither faces a demographic disadvantage purely from being a smaller generation than the one before it.
The more defensible framing is that Generation Z inherits an intensified version of the same structural conditions Millennials navigated, rather than facing an entirely distinct demographic challenge. Housing costs have generally continued rising rather than reversing. Fertility has continued falling in most countries rather than stabilizing. Population aging has continued advancing in the countries where it was already visible during Millennials’ young adulthood. Gen Z is not confronting a new problem so much as a more advanced stage of Millennials’ problem.
Key Findings
- South Korea’s fertility rate fell from roughly 1.5-2.8 during Millennials’ birth years to an estimated 0.7 by 2023, illustrating the sharper fertility decline Generation Z was born into compared with Millennials.
- Global fertility fell from roughly 3.3 to 3.7 births per woman in the early Millennial years to approximately 2.2 by 2024, well into Generation Z’s lifetime, with the share of countries below replacement fertility roughly doubling across the two generational windows.
- OECD average age at first birth rose from 28.6 in 2000, reflecting largely Millennial parents, to 30.9 by 2022, increasingly reflecting Gen Z parents, indicating continued rather than reversed delay in family formation.
- Both Millennials and Generation Z are estimated at close to 2 billion people globally, meaning neither generation is meaningfully smaller than the other in absolute global terms.
- Sub-Saharan Africa’s share of global births rose across the two generational windows, meaning a larger proportion of Generation Z than Millennials was born in the region, while East Asia’s and Europe’s shares continued shrinking.
Conclusion
The most accurate way to compare Millennials and Generation Z demographically is not as two generations with fundamentally different experiences, but as two snapshots of the same ongoing global transition, taken roughly fifteen years apart. Fertility has kept falling. Housing costs have kept rising. Population aging has kept advancing in the countries where it started earliest. Generation Z did not inherit a new demographic world from Millennials. It inherited a more advanced version of the same one.
Frequently Asked Questions
1. Which generation is larger, Gen Z or Millennials?
The two are roughly comparable in global size, each estimated at close to 2 billion people, since global annual births have remained relatively stable across both generational windows despite falling fertility rates per woman.
2. Who is delaying adulthood more, Gen Z or Millennials?
Available data suggests Generation Z is continuing or extending the delay pattern first documented at scale among Millennials, with rising co-residence with parents and a continued increase in average age at first birth through 2022, well into Gen Z’s prime family-formation years.
3. Do Millennials and Gen Z have different fertility patterns?
Millennials, largely past peak childbearing years, show measurable rising childlessness in several countries. Generation Z’s eventual fertility pattern remains substantially unresolved, since much of the cohort has not yet completed, or even reached, its prime childbearing years.
4. Which generation faces greater population aging?
Generation Z is entering adulthood in countries that have aged further than they had during Millennials’ equivalent life stage, meaning Gen Z generally faces a more advanced version of the same population aging challenge Millennials encountered a decade or two earlier.
5. Why does Pew Research Center treat generational boundaries as conventions rather than hard lines?
Pew has stated that generational cutoffs are useful analytical tools for grouping similarly sized cohorts for research, not evidence that a particular birth year produces a sudden shift in attitudes or behavior, since individual variation within any generation is typically larger than average differences between generations.
6. Is South Korea representative of the global fertility trend between these two generations?
No, South Korea represents an extreme case. Its fertility decline has been faster and deeper than the global average, though the broader direction, continued fertility decline from the Millennial to the Gen Z era, holds across most regions, just at less extreme magnitudes.
7. How did the 2008 financial crisis affect Millennials differently than the COVID-19 pandemic affected Gen Z?
Both events disrupted labor market entry for the respective generations at a formative stage, contributing to delayed homeownership and family formation in both cases, though the specific economic mechanisms, a financial and housing market crash for Millennials versus a broader economic and social disruption for Gen Z, differed.
8. Where were most Millennials and most Gen Z members born?
Both generations are concentrated in South Asia and Sub-Saharan Africa, but Generation Z shows a somewhat higher concentration in Sub-Saharan Africa specifically, reflecting that region’s slower fertility decline relative to the rest of the world across the intervening years.
9. Will Gen Z have fewer children than Millennials?
This cannot yet be determined with confidence, since a substantial share of Generation Z has not completed its childbearing years. Continued increases in average age at first birth through Generation Z’s early adulthood suggest at minimum continued delay, which historically correlates with, but does not guarantee, lower eventual fertility.
10. Is one generation’s demographic situation genuinely worse than the other’s?
Not in a way that survives close comparison. Each generation faced a major economic disruption during its formative adult years, and each inherited advancing population aging in the countries where it was already underway. Generation Z’s version of these challenges is generally a more advanced continuation of trends already visible during Millennials’ young adulthood, rather than a distinct new problem.
Sources
- Pew Research Center, “Where Millennials end and Generation Z begins,” January 2019
- Pew Research Center, “Demographic Definitions”
- OECD, “Society at a Glance 2024: A spotlight on fertility trends,” OECD Publishing, 2024
- OECD, Society at a Glance 2024 Country Notes, United States and Canada
- United Nations, Department of Economic and Social Affairs, Population Division, World Population Prospects 2024
- United Nations, World Fertility 2024 report
- Statistics Korea, national fertility data, 2022-2025
