No real cutoff for me. I think of myself as, well, me. I guess ‘woman’ would be what I use. (I’m 27, btw, in case that’s a relevant data point.)
But mostly it’s contextual, based on what I use first - I also pair words like Jomo described above. However, I pair ‘girl’ with ‘guy.’ (No offense to Frank Z, but ‘guy’ and ‘lady’ don’t seem to go together in my mind.) So if I start out talking about the males of a group, and it’s an informal situation, I will most likely use the word ‘guy.’ After that, any females in the group will be ‘girls,’ no matter what the age. On the other hand, if I start out talking about the females, they will be ‘women’ and the males will be ‘men.’
But then, I dunno, I find the whole business weird, anyway. My Loved One and I went out to dinner a few weeks ago, and the waiter said ‘Hi, girls, and welcome to the [restaurant]! My name is [name] and I’ll be your waiter…’ He trailed off, realizing some had gone wrong; we were nonplussed, and looked it, because waiters normally say ‘ladies’ in that speech. This conversation then ensued:
Waiter: Oops, was that offensive, me saying girls?
Me: Nope.
Waiter: Because normally women take it as a compliment. I guess you’re not that old yet, huh?
Me [trying not to laugh]: Not yet, no.
Waiter: Oh. I didn’t mean anything bad. Mostly they take it as a compliment, but some really young ones take it as an insult, but I just figured, when I saw you…[he trails off, realizing there’s no good way to finish this sentence]
Me [definitely laughing now]: No, no, it’s okay. Really! But I think you’d better quit while you’re ahead.
Waiter: Okay. Wait, I haven’t done the wine yet. [Stands there for several seconds, looking lost.] Um. Can we just start over?
Me and LO (who had sat like a stone up to this point) together: Sure.
Me: Probably for the best.
And he literally turned around, walked ten feet away, came back, and started again at the beginning, this time with ‘Hello, ladies, and welcome…’
That place normally has incompetent service, but he was a good waiter as far as order-taking and getting-food-out and so forth went. So I figure, either that was a great act, or he really didn’t know why it’s better to say ‘ladies’ in situations like that - most likely the former. But if the latter, I think now he’s learned.