Legal age for drinking in the UK is 18, and Kitty made a reference to being old enough to drink in Excalibur #91, so think it can be accepted that she was 18 when she started seeing Pete. It's not like aging in Marvel comics has been consistent. It's worth noting that the author said she was at least 18, the editors at the time presumably had no problems with depicting her in a sexual relationship without being worried about alerting the moral majority, so she was, to all intents and purposes 18 for this period.
Of course, later on there was a throw away line in an Alan Davis issue of Uncanny X-men which stated she was 16, while Claremont stated and showed she was below legal drinking age in the US in X-Men Unlimited. In the latter respect, she could still be 18/19 and the idea that she's only about 17 NOW strikes me as unlikely.
So, really, there's little point of trying to keep track or expecting consistent aging, but for the purposes of these stories, when she was with Pete, she was 18.