I’m 40, I ride a hardtail and I **** my knee up nearly three years ago.. No physio or injections seemed to improve it and I was eventually down to riding about once every five weeks due to the pain..
wierdly I discovered that short regular rides fully laden with trailer and two kids, three or four times a week, short rides of no more than an hour or two with as many outrageous hills as poss has cured the problem..
I now find that I can ride pretty much whenever I like, over whatever distance and terrain without any undue pain..
So I guess my advice would be mtfu, ride more and build some muscle