I want to stress the importance of being young and technical. Young people are just smarter. Why are most chess masters under 30? I don't know.
Nothing against 22-year-olds, but here's a more likely explanation: if you have both the virtues of being smart and having lived past your early twenties, you probably have better things to do than go to work for a 22-year-old at a startup. Then again, you can't fault Mark for fluffing his employees.