There may be some of that, but at least as importantly: as people get older they tend to be less delusional about how brilliantly successful their crazy idea is

There may be some of that, but at least as importantly: as people get older they tend to be less delusional about how brilliantly successful their crazy idea is going to be. By "not delusional" I mean "they have a statistically valid estimate of the likelihood of success." That means they don't found companies as often. This is not a criticism of 20-somethings who leap into startups--there is no better time to do it--particularly because some of them succeed, and then we all get nice things. But it is in the nature of startups that the odds of failure are so high that the kind of level-headed, staid, steady, sensible person you'd really like to be running a company is the kind least likely to be running it.