Funny thing, that. I was thinking about this topic recently, as I don't feel like I've exactly "accomplished" anything worth talking about up to this point in my life (and I'm 39). But, that doesn't bother me because, the way I see it, everything I've done in my (adult) life up to this point has been taking steps along the path to this startup that I'm working on now. Or, to put it a different way, I've actually been building Fogbeam Labs since I was 19 or so, even though it's only existed as a legal entity for 2 years or so.
I'd love to hear some YC applicants chime in and share their personal "what's the most impressive thing you've done" stories. If I had to pick one for myself, I'd probably go with one of these:
1. I was formerly Fire Chief of a volunteer fire department, and one of the youngest people to hold that position in my area.
2. I was one the youngest, if not the youngest, Firefighting Instructors certified by the NC State Fire & Rescue Commission.
3. I ran for Lieutenant Governor of NC, appeared in a statewide televised debate with the other two candidates and got about 125,000 votes (roughly 3%).
What I can't claim is to have achieved anything specific in terms of building a previous startup or having any particular financial return, blah, blah.