People can change, but in all honesty, they rarely do. People don't like change. I'm saying that as an assumption that you don't know this person well; it's best to stay away until you get to know them enough to truly determine what kind of person they are (as of this moment).
Honestly I have to disagree with the first part, people defiantly change from 12 years old, to 18 years old. Hormones are running rampant when you are that young and it's hard to control yourself. I do agree though that you shouldn't pursue a relationship with someone unless you know more about them, but I feel that's true with everyone, not just someone who had sex when they were 12.
Edited:
More spot on, how would she do it? Of course it cant be with a boy of the same age, as it would both impratical and awkward. I do wonder.
You'd be surprised of what young people are capable of.