Recovery is important at any age but older athletes may need more time to recover.
It depends on the activity. Running, being high impact requires more than cycling. (Depending on distance/time)
A reliable indicator is resting heat rate. If it's elevated more than 10 bpm, take a day off or at least an easy day.
Attitude towards an upcoming workout is another indicator.
Keeping a training log can also help spot long term trends as I did when I started mixing in cycling.
"To give anything less than your best is to sacrifice the Gift."