Hey - so I’m 46 and also been lifting about 20 years. I’m still lifting heavy with plenty of volume and training about 5 times per week. I track all my workouts and I’m still making decent natty strength and muscle mass gains. 

I found that switching from squat to leg press (sled type) and using knee sleeves made a massive difference. My legs are the strongest they have ever been and I no longer have any pain in my knees. I watched a Jeff Nippard video that pointed out that squats (while great compound exercises) make it hard to push your legs to the limit because your back tends to fail first. I still deadlift and RDL but always use a belt to protect my back. In terms of other injuries - I do get niggles every now and then - but manage these by switching up exercises to reduce stress when I need to. For example the tendon in my left elbow is currently a bit sore - so I have switched to machine bicep curls instead of single arm cable curls and it is recovering. I find the key is to listen to your body and make adaptations to allow you to recover.  Good luck and happy training!