Few researchers would dispute that, in the immediate term, being relatively bigger, quicker, smarter, and stronger is a good thing. Repeatedly, the studies have found exactly that—older kindergarten students perform better on tests, receive better teacher evaluations, and do better socially. But then, something happens: after that early boost, their performance takes a nosedive. By the time they get to eighth grade, any disparity has largely evened out—and, by college, younger students repeatedly outperform older ones in any given year.

Why would that be the case? It all comes back to that relative difference: if you are always bigger and smarter, you may be more likely to get bored, and to think that everything—learning included—should come easily. You don’t have to strive and overcome obstacles in the form of older, more developed kids. If, on the other hand, you’re on the younger end of the spectrum, you are constantly forced to reach for your limits. And unlike in sports, where physical size often plays an undeniable, difficult-to-circumvent role in your eventual success, in school a physical disadvantage can turn into an academic advantage: children may learn to compete where they can succeed, where their persistence and attention can accomplish what their physical size may not.