In my experience anyone playing singles tennis who is much older than their opponent (and still competing) is likely much better technically. I'm not sure the active/non-active argument holds that much weight: a lot of footballers' running is fairly low intensity (they also pretty much stand around at times - particularly strikers, centre backs and obviously keepers), and they get a 15 minute break at half time. The active side of tennis is all high intensity - you can't saunter around to retrieve shots.