Looking around the industry lately it seems that you can't do a startup unless you're fresh out of college with not a care in the world.
On the other hand, if you actually have industry experience (even if you are under 30), you can't "drop out" of "real life" because of implicit responsibilities (socially - such as dependents, and financially - such as rent/bills).
Off the top of my head I think only 4 out of 36 startups in the current batch are fresh out of college.
Presumably YC only works for US companies, as people from outside the US (Canada, for example) wouldn't have the permission to work in the US for the three months?