User blog comment:Merlaa/Where did Merlin go?/@comment-107.10.244.141-20121226223813/@comment-5674726-20121227005205

 The thing is though that if Merlin is immortal then how to did he grow from a kid to a man and to an old man?

Immortality means that he can live forever, not necessarily that he wouldn't age. It's possible that he aged to eighty or thereabouts, and then continued to live on after that.