I'm totally scientific about it. I go by my TrainingPeaks Performance Manager Chart. I do what the numbers say I can do, and sure enough, science works. My double-check on that is HRV and morning resting and standing HRs. I especially watch my HF power (parasympathetic system) in my HRV app. I track my training stress with a PM on the bike and a HRM for all off bike training. Works like a charm. Like you say, over 70, and things change so fast that it's hard to keep up with it, in terms of what to do tomorrow morning. Fortunately, right along with these physical changes are the symptoms of them, like FTP and LTHR. Keep track of those, and CTL and TSB will stay valid numbers, though one's permissible continuous CTL does drop with the years. It's all quite noticeable in the HRM and HRV monitoring though.

I've been taking 1-2 rest days/week, which is what I've always done. My weekly mileage is down, but for the past couple months it's mostly because I've got this horrible saddle sore I can't so far get rid of. Another appointment in a couple days. I've taken up running in response to take up the slack. I've been keeping my training stress score (TSS) in the 400-500/week range. I went out for 1:06 of intervals and hard JRA today and got a TSS of 106, IF of .98. That's a good workout at any age, just that my watts and miles are lower. Yeah, I'm slower, but I try to work just under my limits, just like I've always done. As you note, the trick is to know where those are. Science helps.

People will tell you how many miles and thousands of feet they are riding and I ignore all of that. Everyone's different. Asking others about their stats and days off, etc., maybe isn't even a good idea. Too depressing sometimes, and too encouraging other times.