She turned 16 in the mid teens of excalibur, when the restof the team was lost cross time. Courtney Ross was Kitty's "Guardian" and by the late thirties, they were parodying Saint Trinians where Kitty was (literally) running around in plaid short skirt and knee socks playing field hockey.
They jumped ahead almost 10 years out of no where, when she returned to Xavier's school around about the same time they were keeping Sabertooth in the basement.
Illyanna might have been the same age Kitty at "some" points in their friendship, but thegirl travels through time, and lets remember that this version of Strange she's studying with is from "just" after Giant Size X-Men #1, that he's met Piotor but doesn't know about the (80s version) New Mutants whch takes us back to Kitty pride being 13 and a 1/2 as a chronology indicator.
The "age" thing doesn't bother me.
This is just purely about "shipping".
Although is Stephen's actual "girlfriend" at this point in his timeline Clea or Morganna Blessing?