There was this whole thread on how Adam cannot die/age because his cells regenerate too quickly. The reason why you age is because your cells slowly die, but because he can regenerate them, he cannot die of old age..
Claire has the same ability. Rapid cellular regeneration which will prevent her from getting old. But maturing (ie puberty and such) is a natural process that her ability cannot stop, because it has nothing to do with cells dying.
So she will continue aging, until she's fully grown (around 20 plus I guess) like Adam, and then she'll probably remain that way for ever...
Until someone mutilates her brain, that is. I'm just kidding