There is bound to be some biological change, but as crystalstars points out, many trained singers age gracefully though they do not sound *exactly* same as they did years ago.
Hmm, the only singer I can think of immediately who at his current age (70's) sounds similar to his prime is Dennis DeYoung.
Some singers are just blessed with good genes. Much of it is luck.
I want to speak to this "genes" and "luck" idea. Any professional singer who has come as far as James has "genes" and "luck". That's merely speaking to the level of natural proficiency a singer has that is born not made.
The rest is management. Singing is an art but it's also physical. Think of it like dance. If a dancer doesn't continually train, then their body cannot move well enough (despite being born with talent) to be able to express the artistic messages they wish to interpret or the music demands. In the same way a singer needs to manage their physical training (as it relates to the voice) to continually meet their demands. The voice, like a dancer's body, needs to be exactly as flexible and strong as their art form demands it to be. For James, those demands are extraordinary. As was already mentioned, DT music is far more demanding than most because of its complexity. So his training and maintenance needs to meet the occasion.
Also, and this is very important and I think many forget the seriousness of this:
He sustained a significant injury. A vocal hemorrhage (whatever actually caused it notwithstanding), nevermind one that was not properly cared for in the beginning (he should've had surgery and six months of vocal rest), is a severe and even career-ending event for most. As such, it's like our hypothetical dancer having broken an ankle or knee and then always having to be mindful of it. Maybe they're always in danger of reinjuring and need to take extra precautions and extra physical therapy. Maybe you can even see the precarious lack of balance at times on that leg, even though they continue to dance. This is what James is dealing with, with his voice.
As such, it is no surprise that he's often skating on thin ice vocally. This doesn't make him a lesser singer than others who have never sustained an injury. It makes him a quite tenacious and admirable singer for continuing to do what he does. It's unfair to compare him to singers who have never been injured. They may be dealing with simply the ravages of time but I assure you James is dealing with a lot more than just that.
I do hope that especially now, he gets the care and good advice, training, and vocal therapy he needs. His is still, when he's at his best, the most objectively beautiful tenor voice I have ever heard. Maybe not the biggest, the strongest, or the most range-y of all time (Tate, Dickinson, Kiske, I'm looking at you), but James's superpower as a vocalist is
beauty. The natural beauty of his voice is his blessing of "genes" and "luck".