At 38 I've trashed both my knees after years of hiking, construction, and many years in the airline biz up in Alaska. Running up and down the ramp at a float plane dock with 70lbs at a time will take its toll. Worst is my right knee. I'd taken a spill on a steep slope in high school and kind of tweaked it, but nothing major. A few years later when I was in college, I really screwed it up. Walking down a 30 degree slope in the dark and stepped into a 2' deep hole. My entire upper body momentum flipped me into a somersault down the hill, hyperextending my right knee to the point that it dented the end of my femur when the joint slammed back together. The tendons and ligaments healed up, but here's where it gets weird. Every so often, a small piece of cartilage will break off because of the dented portion, and float around in my knee joint. It feels as if my knee is going to blow out at any moment, feels unstable and weak. When I went to the doctor because of it years ago, he told me that strength wise, my knee joint was fine, but that bit of cartilage makes it feel unstable (and really f-ing painful at times). It has to wear itself down and be absorbed. Over time with more exercise and diet, it has improved. One thing to consider besides Glucosamine is that if you are having an issue with the synovial fluid in the joint, it is not lubricating the cartilage like it is supposed to. Making sure you are getting enough sulfur in your diet is important for joint health (MSM, onions, garlic), and Hyaluronic acid (a component of synovial fluid) can be supplemented as well.
http://en.wikipedia.org/wiki/Synovial_fluid