Because people who become high-achieving outliers tend to distinguish themselves in their adolescence. It's the start of a virtuous cycle where talent + hard work leads to admission into elite institutions (universities and, later, companies) which lead to positive network effects which amplify the ongoing application of talent + hard work.

It's rare for a previously unremarkable 36-year-old to suddenly turn in to Jeff Bezos (for a lot of reasons, not least that cognitive power peaks at age 25 so learning and adapting to novel situations gets harder with age).