Many of the recent Phantoms have been on the young side (probably because so many have moved up from playing Raoul). Hugh Panaro was, I think, 48 when he left. Norm Lewis is 51 or 52. Especially the way he plays the role -- putting body and soul into his performance -- it's got to be rather strenuous. As someone pointed out, he signed a short-term contract and then extended a few times.
I'd LOVE to see Tim Martin Gleason or John Cudia return to the role. I've seen each of them on tour, and they're both fantastic. John Cudia is, I think, in his late forties, however -- not so far off from Norm Lewis in terms of age.
I'd also love to see Sean MacLaughlin (spelling?) or Kyle Barasich get a shot at the role. They were great as Raoul and have beautiful singing voices. To me they also have the right body types -- tall and lean.
Audrey, the Phantom Phanatic, who nonetheless would rather be Jean Valjean, who knew how to make lemonade out of lemons.