When is it time to quit flying?

Just turned 70, been flying since age 16. Wondering about how to tell when it's time to quit. No interest in crashing my GA aircraft and becoming a feature on blancolirio or dan gryder. Fresh BFR, current Medical, current Annual, logging plenty of hours every year. For the last 10 years, have wrapped my pilot certificate in a $100 bill, ready to hand it over to the first pilot who advises me that I don't have what it takes anymore. Truth: I absolutely love flying my single seater, makes me sad to think that one day I won't be fit to act as PIC anymore. Older pilots who have quit, please tell me how you decided that it was time.