Comment by SamReidHughes

(I don't know anything about YC applications but) Because it's a useful context in evaluating the people. For example, how low quality certain aspects of the application could be perceived might depend on the person's age -- one being that a 22-year-old isn't going to have much of a resume. Also, if somebody makes certain kinds of mistakes, it's less of a red flag if they're in their early 20's, than if they're 40 and should know better.