It is simpler than that. There is a social link between age and authority.

For the first third of your life almost anyone you interact with who has authorarive power over you is also older than you. Think parents, teachers, coaches, managers etc.

A VC (or incubator mentor) will most likely have a stronger authorarive stance and feel more comfortable with younger founders. Founders will feel same with younger employees and so on down the hiring ladder.

It requires maturity and confidence to "unlink" age and authority. In a culture as driven by "macho posturing" as the Silicon Valley startup world is, this, in most cases, ain't going to happen.

Thus startups are young.