What is the reason for my knee joint pain starting up 30 years after playing lots of tennis?

Let us to answer. Tennis is demanding sport on both upper and lower extremities, after playing that long period of time some degeneration in the articular cartilages of the joint sequence to excessive stress happened and that cause pain in the joint (knee and others). Good luck.
Wear and tear. This is not an uncommon scenario. Tennis can cause a lot of pounding on the knees, particularly on hard surfaces like asphalt courts. Also the cutting side to side movements can put a lot of strain on the knees. This does not mean you have to give up tennis however. See a sports medicine doctor and they should be able to help you out.

Related Questions

I m 29 years old can I take calcium and vitamin D without going for their levels & without dr prescription I have knee joint pain & I live in sweden

Absolutely, yes! Hi. And in fact, I'd encourage you to. However, please don't expect calcium and D supplements to improve your knee joint pain. Lifelong adequate calcium and vitamin D will help bone health, and you're right around the corner from your peak bone mass in life, after which your bone density, and with fair correlation bone quality, are in decline. Good luck, and I hope your knee pain is temporary!
Only if. Only if you have a baseline blood chemistry panel as there are medical conditions where they can cause problems. And only if you take the recommended RDA amounts. And only if you realize it won, t help your knee pain and you need to see someone for this knee pain.

What do I do about knee joint pain that only hurts behind my knee?

Depends on the cause. Many things can present as back of the knee pain: meniscus tears, cartilage injury, hamstring tendonitis or tears, muscle strain (calf or thigh), arthritis, bone bruise, nerve injury. If the pain does not resolve with rest, nsaids/tylenol, ice, and activity modification, see your doctor.
See your doctor. You could have a host of issues that could cause your knee pain, could be bursitis, tendonitis, arthritis, ligament issues, meniscal injury, sciatica etc. So you may want to see an orthopedic pain specialist that can tell you what is going and help you with the latest treatments and exercise therapies available.