If you have pain when you squat, then something is probably injured.
What kind of pain is it? Sharp, dull? Does it come up slowly and get worse the more you squat (or keep playing)? Or does it only hurt on a specific movement and goes away straight after?
As for playing, as long as you don’t feel any pain while playing, it shouldn’t be a problem. If there is pain, you should stop playing or it could get worse. See if you can’t get a doctor or physio to take a look at it.
The exercises to strengthen the knee would be squats and similar exercises. You should find light exercises that target the knee joint without hurting it, like leg extensions with little to no weight. You can then slowly add more weight, and work your way up to a full squat.
Also, at 109 years old, it can be very difficult to keep your joints strong
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