I am 55 yo have chronic soreness in L ankle, 10 weeks post meniscus surgery on L knee, chronic but mild soreness on front of R rotator cuff, and light soreness in R elbow on occasion.
Here's my wisdom for what it is worth:
1. Take off days - I like to take 2 or 3 days per week off from tennis
2. Ice the sore bits after tennis
3. Work out regularly - including strengthening and stretching. Light weights for upper body, ankle weights, 1/2 squats, heel lifts and several lower body exercises. see "thrower's ten" and "knee rehab" on the web for exercise details. STRETCH afterward every time. I work out mostly on tennis off days
4. Lower tension on strings and get a comfortable racket - I look for stiffness ratings below 65. Maybe you could go stiffer if a volkl or pro kinnex racket which are good on comfort
5. Use good technique - use body rotation for power and not arms and wrist
Of all these, simply resting between playing days is the best.