I think this is related to an implicit assumption that startups are for twentysomethings, where you have to have started early in order to have much experience

I think this is related to an implicit assumption that startups are for twentysomethings, where you have to have started early in order to have much experience at all. Maybe it's just me, but I think the startup culture has an emphasis on youth that leads people to wish they "started younger", instead of "finishing older" -- not that you're ever really finished. Perhaps people feel like 35 or 40 is too old to start work at a startup. I wonder if this pressure is particularly noticeable for women, since it seems like most other pressures of aging are exacerbated for them. Maybe the only difference between a 25 year old with 10 years coding experience and a 35 year old with the same, is that the former is more likely to be accepted into the startup community.