A year ago I would have said the same. I do think that it was possible that I was suffering from a post-viral fatigue. Having got in better shape, and done a bit more fitness work - and crucially being illness free for a bit, I would say that I'm much improved.
I'm fifty eight shortly. I wouldn't say that being over fifty, as of itself, is reason to be unable to manage ongoing physical exercise, and have periods of relative incapacity. Diet, sleep, and regular exercise should be relatively thought out.... but for me if you are doing that, but can't, there's a possible sub-optimal health issue.