How old is Erik/The Phantom?

It depends of the version. If you read Leroux or Kay, the Phantom is AT LEAST 50 years old. We are never sure of it, since he has only stated he has started his Don Juan Triumphant 30 years before... So yeah, at least 50. In ALW... when it comes to the musical, it depends. There were 20-30-year-old guys such as Ramin Karimloo who have portrayed Erik, others being around 50-60 like in the book...
In the movie... Mme Giry looks to be around 40. I would say Erik is about 30-35. Gerard Butler's age when they did the movie, actually. ;)