She joined the X-men at 13 1/2. She turned 14 with the X-men in space(around 164, iirc). Jim Shooter nixed her relationship with then 19 year-old Colossus during Secret Wars. She turned 15 with Excalibur (#26). Kitty claimed she was old enough to drink in Excalibur #91, which by Scottish law made her at least 16.
Warren Ellis has said that he thought that she was 18 when he wrote her in a relationship with Pete Wisdom. Claremont de-aged her in Uncanny, where she claimed she was only barely 16. Claremont then had her appear in Unlimited as being a young college student of 18 or more, able to vote and work at a bar but not able to actually drink.
Joss Wheedon didn't change her age at all, really.