Where has it been said that she would be in her teenage years ? I seem to have missed it ?

Yep pretty young things clearly are a trend in the show and I don't really know why I counted on Moffat to change it :/

Then again I'm all for some change (and I think there will be changes with the Moffat era), but lots of those pretty young things were also amazing characters in their own right so I don't really see the problem if they're yummy as well ;)

We might not know all the plans and maybe she won't be straight the only companion on board or like, die after 3 episodes ! lol