I've gotten to a point where I'm more frequently associating with people (including 'females') in the 25-35 age range and have had this debate internally. I've started using 'lady' for this very purpose, though I realize it's not as socially common. I might recount an interaction by referring to 'A guy' or 'A lady' I met (rather than 'girl'). Personally, I'd love for it to be socially acceptable to refer to people with words that don't presuppose age (or gender, for that matter) but that time isn't coming anytime soon.