Hrrm. I think this is likely a case of 'people being confused about the age of teenagers'. I'm pretty sure that Kitty was, what - 14? - when she came to live with the X-Men, and given the extremely slow march of comic book time, no more than about three years or so could've passed between then and the Kitty/Wisdom years, in my opinion (especially considering that her characterization in Excalibur PRIOR to that was as a 17-year old at MOST - she always struck me as more like 16). This is also taking into account that, a number of years later, after Colossus's brief 'death', she briefly left Xavier to go to a standard college - and while doing such a thing in your twenties is far from unheard of, in the one issue I have of that event, she was going out for pizza with a group of other students, mostly upperclassmen, and she looked there like she was younger than them - I'd have said 19 at the most, while they were all in their early twenties, 22 or 23. So I'd say that, if she and Wisdom WERE having an adult relationship, it was more a case of her going 'eh, close enough', and fudging her age. (She still shouldn't be allowed in a bar, though.)