@Morgan97 well accrdg. to the movie, the Phantom/Erik is as old as Madamme Giry (Meg Giry's mother) and was really old when he met Christine Daae and fell in love with the latter while teaching her how to sing.
I think, as the story implies, the phantom is somewhat an immortal thingy and all. So, i think that he never aged or stopped aging. And accrdg. to the movie, the gypsies than enslaved Phantom/Erik referred to him as "the Devil's Child"....
I hope this answered your question... :))