As you get older you will naturally lose some of your mobility and speed. Should you quit? No! Should you go to the gym? Possibly! But, in reality, if you concentrate on all the balls that you can stop and hit and all the balls you can reach in three steps or less, you will be excellent!
Quit worrying about the few balls that you cannot get to. They will only be a small percentage of the reason why you lose a match. Remember to always go to the ball! Don't wait! Always think that every ball is coming to you. Never assume that when you are at the net and your partner is behind you, that your partner can hit a better shot than you. The player nearest the net is the "Boss" or "Captain." Try to hit every ball! Do not play to avoid the ball! Get out there and move. The movement is fun and very healthy for you! Don't act your age!
Good luck on the courts!