I'm 21...based in Canada.
I've been:
Canadian Plus Gold from 1998-2000
Aeroplan Elite from 1999-present
Starwood Gold from 1999-present
While I do get some questionable treatment because of my age... on the whole I find that service staff usually don't put too much weight into how young I look. I always try to dress well even when I'm not travelling, so perhaps that could have something to do with it. Rude service because I'm young is one thing I will not put up with. Luckily I don't have to deal with it too much.