The Pokemon girls stopped looking 10 since Kris.
In other words, they never did look 10.
Which I agree with, but this one even less so. I'd mark her at a little less than Clair's age, maybe about 16-18.
By comparison, I'd say May from Ruby and Sapphire looks to be between 13 and 15.
EDIT: You know, it just occurred to me. If that's really the final design, maybe the older look is an indicator that the Player Character this time around is a college student. It be a much easier way of justifying their random association with the region's top Pokémon researcher. What if you're the student of the Professor in this one, and he assigns you a starter pokémon as part of an extracurricular research activity you've volunteered for? The college itself could teach classes about several different pokémon-related activities/potential careers, such as battling, breeding and contests. The Gym Challenge would be a different class assignment, though the Professor would use it as an excuse to have you monitor the growth of your starter.
Thoughts? Probably won't happen, but I think it's an interesting concept.