As a YC alum (and thus someone who's experienced the folks involved, though I'm not an expert on this), I think it's much more likely it's mostly used to gauge

As a YC alum (and thus someone who's experienced the folks involved, though I'm not an expert on this), I think it's much more likely it's mostly used to gauge demographics or potentially to increase diversity rather than reduce it as seiji suggests below. Edit: to clarify, I think you incorrectly assume they're asking for age to filter for candidates that are younger. I don't think they use to filter at all, but even if they do, I would consider it highly improbable that it is to target a specific range.