I think it's more that HN commenters skew younger, so you see a peak in mid-20's.
But anyway, I disagree with lemonade stands/mowing lawns not counting. You can learn a lot from the simplest business. And that knowledge (hopefully) causes you to avoid making the same mistakes later when there's much more at stake. That, and remember that most startups aren't financially successful anyway.
They do count in this sense, but the term start-up is being stretched thin (not only here on HN). I guess a more appropriate question to these answers would be "what was your first business experience?"
But anyway, I disagree with lemonade stands/mowing lawns not counting. You can learn a lot from the simplest business. And that knowledge (hopefully) causes you to avoid making the same mistakes later when there's much more at stake. That, and remember that most startups aren't financially successful anyway.