Jubilee

Jubilee is a Marvel Comics mutant introduced in Uncanny X-Men #244 (May 1989)

=Teen=

13
It isn't clear which issue first established her age as being thirteen when joining the X-Men.

Unknown source
One known instance is when she is older she refers back to it when speaking with Wolverine though:



1994
Generation X Collector's Preview. Rescued Wolverine from cross in outback

16
Generation X #65-69

17
Turned into vampire?