As some of my students, who play doubles, get older, I hear the same story. "I'm too old," "I'm too slow," I don't see the ball very well," etc. Trust me, you are not the only one thinking these things or having those problems.
Where to start? I call the solution to these problems "your first step." You might have gotten into the bad habit of only trying to hit the ball when everything is perfect. This is not going to help. Every time the ball is hit make a step or movement in the direction of the ball, even if it is ridiculously far from you. Remember, the opposing net player is not sure if you will make the shot or not. So, even if you do not hit the ball, you stop the opposing net player from attacking the net position so soon. Very valuable!
Part of the fun of the game is exercise. Move to the ball. Every ball! Never stay in the same place two shots in a row. Picture where your opponent's shot crosses the net and move aggressively to that intersection. When you receive a ball to your forehand, take the non-dominant hand off the racket and let your opponents see that you are attempting to hit it.
When you start playing a doubles match and the ball goes by you and all you do is turn your head and look back at your partner, no one wants to play with you. Make a step or move to every ball, not just the "perfect" ball. Even if you do not hit the ball you should move and adjust your position for the next shot.
Do not play like you are "old." The movement will keep you young!
Good luck on the Courts!