I would say if you're 3.5 and under...it's probably a lot easier to play tennis for longer.
The higher up the chain you go and the more athletic and physically demanding the points, it's obvious more is asked of your body.
If you started young and are playing the bulk of your tennis on hard courts to boot...I'd say it's NOT really a sport for a lifetime.
Doesn't mean you shouldn't try, though!!!!
Mental game? That's for people without sick timing on their forehands.