Another reason many startup founders are young - salary
A 22 year old founder can take a low salary, and not be too far from the market rate.  What if the market rate for the founder is 300k?  There is no way anyone would take anything near that much salary at a startup.  The older you get, the bigger the sacrifice becomes.  Now, maybe the older founder gets more equity, but any significant amount of equity is still pretty much an all or nothing payout.  The younger founder gets a better deal - salary closer to market, same all-or-nothing equity.