It's when Kitty joined Excalibur, I believe that Warren Ellis was writing at the time. And I'm also pretty sure that Kitty was meant to be at least 17/18 in that series (she was in a pub on at least one occasion). It's just that when Claremont starting writing her again, he ignored her age in Excalibur and started writing her as a teenager again (because he preferred her to be that age).