An opera professional said to me recently: "No operatic tenor would dream of performing one of their old songs in the same key that they first recorded it but Paul McCartney does it all the time."
We notice it in McCartney's case because he's so clearly trying to emulate his younger self in every respect. Most of his contemporaries have stopped trying. Robert Plant's still got a good voice but it's not the same one that he used to front Led Zeppelin with, which probably explains why he hasn't rejoined the band. Bob Dylan's reed is broken but can still sell a song somehow and he probably likes sounding like an old man. But when Joni Mitchell re-recorded "A Case of You" in 2000 you could tell the way she sung the word "Canada" that she couldn't get near the bell-like top note of her 1971 recording. It seemed like a terrible capitulation.
James Taylor is one of the few people who was performing in the sixties who still seems to sing in the way he did in his pomp. Nick Lowe sings better than he did in the days when he was having hits. Both of them seem to sing their age. They record quietly, which makes a a difference. Georgie Fame's almost seventy. If the evidence of his new album Lost in a Lover's Dream is anything to go by, his voice has lost nothing since the sixties. He doesn't have to stretch because the most he's competing with is a guitar and bass and the songs, standards like "Cry Me A River" and "My Foolish Heart", are the kind of saloon-scaled material that are well within his range. I even like this one, which is about winter sports. Listen for the plosive at 3:17.