I agree they can be the perfect environment for a jack of all trades. But they too often (not always, not uniformly) have recruiting, interviewing, and hiring practices that screen out older people, or anyone who isn't just like the team. That may be discrimination, or (more likely) just the normal human tendency for people to associate with other people more like them than not. By the same token I'm not going to fit in at a skateboard park or a club for video gamers.

Age discrimination in the software business is a real thing. There's a widespread (and I think wrong) notion that older people are stuck with outdated skills and can't learn anything new. Young people can have a lot of hubris. I generally excuse that as the inexperience and over-confidence of youth (I am the parent of three millenials).

A team of 20-somethings may not want me, and they may be right that I won't fit in. I think I might add some experience and maturity the team lacks, but too much friction in the team will work against everyone. Team dynamics are a first-order driver of productivity, so it's up to me, as the outlier, to persuade the team that I will fit in well enough to add value, even if I'm not going to play foosball or decorate my cube with anime (exaggerating for comic effect, but also directly from my personal experience).