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.
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.