An older Phantom can be the tragic romantic figure if you're into that sort of thing.
In more seriousness, Phantoms like Michael Crawford gave me the impression that in the context of the show, the Phantom's "sexiness" originally came from his mysterious nature, his gorgeous voice, his strange charisma and intriguing gestures--all of which disappear as soon as the mask comes off to reveal the real man behind all the smoke and mirrors. Aside from that, it's also possible to play up the masked Phantom's "sexiness" with more middle-aged portrayals by going for the silver fox bachelor approach vs. going for the young and hawt angle that ultimately defeats the point of the Phantom's tragic situation. (You're supposed to sympathize with the Phantom because he's UGLY underneath that mask, Mackintosh!)
"Was uns befreit, das muss stärker sein als wir es sind." -Tanz der Vampire