63 y/o here too. For two or three years I've used a heart rate monitor on most rides, runs and walks. And a heart rate variability app.
I can't say that HR and HRV data is reliable every day and workout. I mostly use it to look for patterns over time and many workouts.
As others noted, heart rate can vary depending on rest, diet, medications and supplements, alcohol consumption, and other health issues.
My maximum HR is 173, consistently on tests. I usually avoid pushing more than 160 bpm most workouts, since it takes me longer to recover now. I used to do workouts that might be called mostly tempo or sweet spot -- mostly fast (for my age and condition) group rides -- but wasn't making much progress and probably wasn't adding enough variability and getting enough rest. So I took advantage of the pandemic to skip group rides and try other training methods. I found it easy to do hard workouts (per HR data) and hard to do easy workouts. For example, with running per some training recommendations, the only way I can do 80% lower effort is walking. To do those 80/20 workouts, I'm mostly walking with intermittent sprints or strides. With solo bike rides it's easier to finesse by coasting and soft pedaling between harder efforts. It's impossible to follow that training program in typical club rides because every club ride I've done has been a tempo ride, so my HR is 130-150 bpm 90% of the time, with some harder sprints and short steep climbs into zone 4 territory.
While I do record this stuff most workouts I don't rely on it for my approach to a workout. I just go, see how I feel after a warmup and adjust my workout effort to suit how I feel that day. Occasionally a hard workout ride or run corresponds with HR and HRV data, but not always. Occasionally my faster rides and runs are on days when the HR and HRV data seemed to indicate I should have taken and easy or rest day.