In the first movie, teens, yes. But they changed actresses and, I think, ages, in between. Wasn’t she the little girl whom Xavier was talking to in the hallucinations? I didn’t get a good look at the phasing-through-the-bed scene, but the girl in the hallucinations was definitely less than ten.
No, the girl in the hallucinations was nobody in particular. Certainly not Kitty. (She was played by Keely Purvis - who is 12, FTR.)
As I pointed out, the actress who plays Kitty in X2 is 18 (almost 19) years old. Katie (I misspelled it before) Stuart’s bio
I remember hearing that Jubilee (Asian girl with the big earrings) got de-aged somewhat between movies, but she didn’t look terribly young to me in her brief scenes in X2.