[ganley]

Easy: We're expensive, and we don't work 80-hour weeks. (Both generalizations, but both certainly far more true of older engineers than younger ones.)

That said, I've worked for two startups, one when I was 31 and one when I was 38. If you bring something to the table that they need badly enough to be worth the higher pay and lower time commitment, it can happen. As @steveklabnik said, it's just math, though the math doesn't always fall in favor of the young guy. Just usually.