I think it depends on the person. Some people are finished maturing early, and others can be ancient and yet still immature.
I know that Midgardian mortals measure time so because their time is short compared to ours, but the preoccupation with age is a youngling's trait to me. But if society values a woman based on her youth, it makes sense that a woman knows her value. Having said that, I suppose Barton was trying to make a joke, if not a funny one, by calling Christine old?