They're all old as fuck, Yuyuko is most probably over one thousand years old. But the only ones who make fun of this are the people who enjoy the stupid "cirno baka lolol" faggotry et cetera. She retains her youthful appearance, because she's a fucking ghost, breaking news! Old lady jokes my ass, seriously.
This. "Old Maids' Alliance" is a contrived fan gathering for the mature-looking Touhous. Tewi's easily over 1000 years old, but she ain't in the Alliance. Keine looks mature, but she ain't in the Alliance either. Why not? Because overall the fandom (which created this Alliance in the first place) has confused 'mature' with 'old'. But despite the fallacies of the name of the group, no one's complaining because all of the members are yummy eye candy, which is why it doesn't get changed to be more logically correct.