Nope, Warren Ellis has gone on record that as far he was concerned, and he WAS the sole writer of the character at the time, by the time Kitty was in HIS Excalibur she was 18 or 19, and more than capable of choosing her bedroom partner.
I believe Claremont de-aged her again when he resumed writing her, but that was HIS problem, never Ellis'
Given the fluid nature of comic time, you cannot cite a birthday in a comic written something four years previously by another writer as proof of age of a comic character. she could easily have had multiple birthdays between times.