... but by being allowed to be bad at their job at 21, they are more likely to find success later.

This is not just true of CEOs. As a precocious chief sysadmin of a small local ISP at age 19, I was allowed to fully administer things no sane company would allow a 19 year-old to touch: sophisticated ADSL aggregation equipment, BGP-speaking border routers, mission-critical core routers, ATM equipment, and so much more.

Not everything I did with this power was smart. A lot of my understanding of what that stuff is was learned "improvisationally", and major local customers, including the county government, paid the price. But what I did do was very successfully parlay this experience into a big-kid job (and a mega pay rise) in the big city, which then begat more such jobs. I doubt I could have rocketed that high, that quickly, if the small-town ISP weren't willing to let ambitious but inexperienced people like me screw up, in exchange for paying them semi-student wages.

The purpose of a startup is not to supercharge the career of its founders.

It's not usually the purpose.