I asked a doctor buddy of mine "When are you too old to participate?" He replied "When it takes too long to recuperate."
Due to knee issues, I rarely play singles anymore, but love good competitive doubles! The week before last, I played 5 sets on a Friday morning. It takes me about 3 days to recuperate (although my knees complain every time I go up or down stairs). I know I'll pay as I get older. I'm just hoping medical advances will out run my playing life.