I doubt anyone really goes "ew so old, so unrelatable, leave this society". There are also graduate-only societies, and societies on topics that tend to be popular around older students. Really, tons of options.
Personally, if I was a 17 year old in a society with a 25 year old I'd immediately start off with respect for them but the rest of the relationship would be the usual.
I think every other answer has good points in it - really, no one can tell the difference between 22 vs 25 y/o, especially if you're trying to keep a "young" look.