Speaking as a 27-year-old, it seems fairly reasonable to me. This is the age where you've had roughly 5 years of career experience in software or web development (if you went to college), and in the adolescence of your career you may have encountered a problem or a market that seems interesting and may also have a desire to be your own boss and avoid the tedium/politics/etc. of the places you've been so far (because, naturally, you won't make the same poor decisions!).

Not sure that I can speak to the prevalence of social startups in this age range, apart from the obvious "kids these days" take on it. Bear in mind, though, that it's not purely a representation of what 26- and 27-year-old founders are doing -- it's also reflective of YC's position.

As a member of that age cohort (but not a founder, much less one funded by YC), I feel qualified to speculate wildly about this!

We were exactly the cohort who got facebook off the ground - I joined in March of '04, spring of my freshman year - so while we may not be "social web natives" depending on your interpretation of such a noxious term, we're definitely well-acquainted with it. Probably well enough acquainted to feel that we know what features are missing, what niches are underserved, or something along those lines, with existing social services.