I feel your pain. I've had knee and back problems since about the same age as you but it was unrelated to cartilage. I'm 21 and this year I found out I had bad cartilage in both elbows and both knees. It was my own fault and I don't really like talking about it because I was being real stupid, but anyway, I can barely walk and can't throw anything or really use my arms in more than trivial tasks without getting sore. I hope you're not as bad off and that makes you feel better. I know when people ask me what's wrong and I say bad cartilage they give me this look and say something like "that sucks" and I think it's rude, but I don't really care. About a month before the cartilage thing I had an epiphany and a change in world view, I used to be really selfish and negative, but anyway, that's pretty complicated, but what I think is important to say is I started exercising and meditating and quit abusing substances and even quit cigarettes in 2 days, coming from smoking 2 packs a day, and after 2 days of exercising and meditating I didn't even want to smoke at all after 2 days, I just felt so good, but anyway, even though I can't exercise anymore meditation REALLY helps. I'm still a lot happier than I used to be, even though I can't use my body that much. I just wanna say that if you get bummed out because you can't exercise, meditation can be really helpful in relieving stress and finding inner peace, it was really amazing the first time I did it successfully, and you don't need to take a class or listen to someone how they say to do it, you just find a peaceful place, my personal favorite is a forest or anywhere outdoors with wind and chirping birds and stuff, and you just lay down, close your eyes, and you can think about anything you want, I usually think about perception and existence and what the things I'm feeling mean, or how they are significant, but you don't have to think of "a candle" or something someone else tells you to think of, maybe what I think of might help, but anyway, you just close your eyes, and focus on what you feel/hear and think about whatever you want and don't open your eyes until you feel a lot better, I usually know when it's time to open my eyes. Despite what some Bob Marley types think about doing drugs, real buddhists don't do drugs, and drugs definitely interfere with meditation, but it still can be worth doing, it helped me quit drugs when I still had them in my system, but it's a lot easier and better now that I don't do drugs anymore. Whatever you do, don't descend into addiction of any kind. That leads absolutely nowhere and will make your life hell. Sorry to write so much but I do feel your pain, and I actually am not sad at all about my condition, I never really even think about it that much, I never complain about it ever, and thanks to meditation this is actually the best year of my life. I hope you don't let your condition ruin you, because not being able to excercise is not that big of a deal, unless you make it a big deal.