She was 13 1/2 when she joined the team. Turned 14 when they were in space fighting the brood. I would prefer if she were still a teen, no more than 17/18, but they've been playing more 20's. She's the Headmistress as the Jean Grey School without anyone ever mentioning how young she is for the role.
Edited to add: I think she turned 18 when she was in the UK with Excalibur and started banging Pete Wisdom.