There's no "number" for any specific person. If you have your ears and they are Ok, frequency-wise, then anything else going wrong, well, "it's all in your head," as they say.
Everyone ages differently, and different parts go at different times. My knees weren't the first to go, but they've been trying really hard for some time! Hearing, technically, probably depends more on choices, poor or good, made much earlier that we can't undo, like closing bars playing in front of a stack several nights a week, working in a shop cutting wood with a radial arm saw and zero hearing protection, etc. Boy, if I knew then... Sounds like you dodged those bullets, so good for you!
I'd try to figure out what you think is worse in your later work than before, and understand why you're are feeling a lapse. Times do
change, and the kind of mix that was great 40 years ago is going to probably miss today's market by a wide mile, though, of course, there are places the tried and true techniques may still apply. (I don't know, as I'm a late *home* recording entrant.) Don't be afraid to ask for comments/input at places like this, or any where else where you hear mixes that sound better than what you are doing.
I'm past 70, and my ears have taken a beating, but I still hear a lot of things pretty well, and I, increasingly, use some of the digital tools to confirm that I'm not missing something, which I might do without them.