When werewolves imprint on someone they still don't grow since they don't have control but the imprinted one does grow (except renesmee) but so wouldn't the girls be old while the werewolf guys stay young?

umm i believe that once the stop having to phase the start to grow again. so if Claire and Quil left fork [when Claire is of age] Quil should stop phasing and start aging again but im not sure good Question

Doesn't it say that they don't past where they fully mature until they manage to control the phasing completly

They can sort of give up being werewolves... "The wolf-men could grow old like anyone else if they gave up their spirit wolves." Its written in Eclipse, chapter 11, at the bonfire, when the legends of the tribe are being told...

Maybe no...because she is an immortal...

No, because it says somewhere in the books,that once they think they are ready they can switch it off.
Obviously they would turn it off when their ages meet.