I was 26 when I first started working with startups, 29 when I first joined a start-up full time, and 33 when I co-founded my first start-up.
Interestingly, at the time, I felt I was “too young” for both my first start-up job and my first time as a founder. I joked (but seriously) about wanting de-Grecian formula to add some grey and look more mature to enterprise and big customers.
That was a long time ago. So much has changed in entrepreneurship.