And what have you done lately?

Feeling good about yourself today? We can fix that. On Wednesday’s Forbes ranking of the 400 richest Americans (to get on the list, you had to be a billionaire), eight were under 40. Four were under 30.

How does one become a fat cat before the hair starts to gray? Do something on the Internets. Of the 20 youngest members of the list, three are from Google and another three are founders of Facebook (along with relatively-speaking mature Web businesses like Yahoo and Paypal).

Of course that latter group includes founder Mark Zuckerberg who is worth $6.9 billion and at 26 certainly acts his age sometimes (if an upcoming movie is to be believed).

That said, the caricature of the nerd-as-tycoon doesn’t completely hold water. Some of the rich kids hit it big in finance – hedge funds were particularly lucrative. Owning a pipeline or two, or an NFL franchise also helped.

And, of course for some there was the time-honored American way of making money. They inherited it.