I never claimed 38 & 39 was old but he's obviously not going to have as much stamina as he did when he was 25 (hence him needing to use oxygen masks throughout the show). That's what normally happens with people; as they age they won't be as fit as they were before. It also applies to sports; athletes will obviously be fitter & have more stamina when they're 25 than when they're 39. It's also important to consider that Michael might've had some issues with his lungs on that tour which would've also affected his stamina.
James Brown may have still sung live when he was in his 70s but he obviously didn't have the energy & stamina he had in his 20s.
Where does this information come from?But that's such a niche example you've chosen just to play devils advocate; exceptions don't disprove rules. I think we can all agree that 99% of 25 year olds are healthier than 86 year olds (most people aren't even healthy enough to live that long). Anyway the main point was Michael was fitter when he was in his 20s than when he was 39 which can be clearly seen in how energetic he was on stage, how little he used playback, the fact he didn't need to breathe in oxygen masks in between songs & how short his breaks were.
Michael Jackson had long breaks and oxygen assistance (between songs) only in the 1993 'Dangerous' concerts, when the stress from the child abuse accusations against him affected his overall health at that time.
In any case, Michael Jackson did not seem to care about his extensive lip-syncing during the HIStory Tour, which apparently was a matter of choice on his part.
He knew that he was about to perform in sold-out stadiums/arenas (during the HIStory Tour), so why to make his life harder by also singing live while executing his dance moves?