Born to lead: The social advantage of being the oldest in your school cohort

This is a summary of the working paper “Who Leads? Relative Age Effects on Social Capital” by Matthew Jacob, a graduate student fellow at AIBM, and Michael Bailey.

For decades, education researchers have observed that children who are older than their classmates tend to do better not just in school, but far beyond it. As adults, those who were relatively older in school are more likely to become CEOs, Nobel Prize winners, and politicians.

The conventional explanation for this phenomenon is academic preparation. Relatively older children enter kindergarten more prepared and go on to achieve better test scores.

But there’s a problem with that simple narrative. Most of the test-score advantage associated with being older fades out by the end of high school. Yet the leadership and career advantages persist well into adulthood. So why do relatively older children perform better?

Our research using data on 33 million U.S. Facebook users suggests the answer is not only cognitive. It is also social. And the effects are strongest among boys:

• Boys who are older than their classmates build 11% larger social networks in high school, an advantage that does not extend to relatively older girls.
• Older boys are 42% more likely to become class president. Girls are more likely overall to hold these roles but receive a smaller relative-age boost.
• Men who were relatively older at school have larger and more connected social networks, and are more likely to be married.
• Boys who start school relatively older are 5-8% more likely to later work in socially demanding fields, including management and entrepreneurship. This difference between older and younger boys is comparable to the gender gap in managerial occupations.

To study these effects, we compared children born just before and just after their state’s kindergarten cutoff date. For example, in many states, children must turn 5 by September 1st to start kindergarten that year. That means a child born on September 2nd enters school nearly a full year older than a peer born on August 31st, even though the two children are only born days apart. While not all parents follow school entry rules, the quasi-random variation in birth dates around these cutoffs provides a way to study the long-run causal effects of being among the oldest children in a classroom.

In the figures below, the vertical line marks the kindergarten entry cutoff (e.g., September 1st in many states), normalized to zero. Students to the right of the line (e.g., those born on September 2nd) are among the oldest in their grade, while students to the left (e.g., those born on August 31st) are among the youngest. We focus mainly on boys because relatively older boys build substantially larger friendship networks in high school, while estimates for girls are close to zero. We discuss effects among relatively older girls on other outcomes where relevant and full estimates are provided in the working paper.

This social advantage persists into adulthood

Most strikingly, these social advantages remain long after high school. In adulthood, the boys who were among the oldest at school are no longer meaningfully older in any practical sense. A few months of age difference matters far less at 35 than at 5, but the social differences endure. Men who were older than their classmates continue to have larger and more connected social networks decades later and are even more likely to be married. The pattern is more muted for women. Although women have more Facebook friends than men on average, we find smaller relative age effects on social network size and marriage rates.

The durability of these social differences contrasts with the well-documented pattern in academic achievement, where the early test-score advantages of relatively older students largely disappear by the end of high school. The social differences associated with relative age, especially for boys, appear to last longer.

Figures 4 and 5

We also find that the social advantage for relatively older men is replicated in other high-income countries with similar rates of Facebook usage, including Canada, Finland, Japan, and the United Kingdom. This suggests that the social advantages of being among the oldest students in a class are not uniquely American, but may reflect a broader feature of childhood social development for boys.

Notably, these countries also vary in the age at which children begin formal schooling—from age 5 in the United Kingdom and Canada to age 7 in Finland and Sweden. The consistency of our results suggest that simply delaying school entry for all boys is unlikely to eliminate the relative age advantage, because some boys will still be older than others within the classroom.

Who benefits from being older?

Our findings contribute to the contemporary debate over when children should enter kindergarten and policies around redshirting. In recent years, more affluent parents have increasingly delayed kindergarten entry, especially for boys, in order to ensure their children are among the oldest in the classroom. The usual justification is academic readiness, but our results suggest that what parents may really be providing is something broader and more durable: a social skill advantage.

Our research also raises important questions about inequality and fairness. If children from more advantaged families are better able to secure the benefits of being older in class, relative age may become another mechanism through which early advantages compound over time. Being older within a school cohort appears to influence who has more friends, who gains leadership experience, and who builds the kinds of networks and abilities that shape their career choices decades later.

The policy lesson is not simply that more boys should be held back. Delaying school entry for all boys would not eliminate relative age differences, because some children would still be older than others within each cohort. A more targeted approach would be to help younger students, especially younger boys, access the kinds of opportunities that relatively older students are more likely to receive: mentorship, leadership roles, and support in building peer connections. Helping younger boys build those connections could reduce the chance that small differences in birth dates become lasting differences in opportunity.