The YC application requires you to list your age, it does not ask if you are vegan. I think it's completely legitimate to ask why age is explicitly included in the application process. I also think it's an interesting discussion whether it is okay that things like requiring relocation unintentionally bias against age (I think it is okay, but it's an interesting discussion nonetheless).

I'll grant you that the title is total click bait, and I don't think it's likely the OP specifically was rejected due to ageism. But I don't think that was his main point.