The obvious examples that support my contention are Apple, Microsoft, Google, and Facebook, but it's not obvious that those are actually representative of the huge diversity of startups. The problem is that I can't come up with any counterexamples.
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[0] https://hbr.org/2021/12/dont-let-age-get-in-the-way-of-entre...
[1] https://www.forbes.com/sites/theyec/2020/06/05/you-just-turn...
[2] https://www.inc.com/jeff-haden/new-research-uncovers-ideal-a...
[3] https://smallbusiness.patriotsoftware.com/average-age-start-...
There are plenty of exceptions, of course. But as a general rule startup founders are young because young people have no responsibilities and the energy to keep going day after day, week after week, month after month, for years on end until they exit. An extra advantage young people have is they don’t know how hard it will be. Older, more experienced people don’t have this tailwind of ignorance.
This was a bootstrap too.
In terms of (much) smaller exits, I'm the child of a startup founder. I was about age 8 and it was sold when I was about 25. It was never more than 20 people, and my dad was President as the founding engineer. It was enough for a solid middle class childhood for myself and a sibling in a VHCOL area and my dad and mom are very comfortable in retirement.
It helped that my mom was stay at home, I think. My dad bootstrapped his company and had to work on it nights when I was young after his paying job before they had enough customers that they could go full-time. They never took funding. He made time for us on the weekends, but my mom did all the weekday stuff