All explainable! He could come into contact with something alien and weird that allows him not to age, or he could glom onto some other people eventually.

Although my guess when writing this is that Adam can look anyway he wants to look, since he's basically not human.

Ha, I think it would have been awesome to bring Ianto back evil in some way. Secretly evil, like being unaware he's like Beth with HIDDEN KNIFE ARMS.