Had cartilage replaced when I was 19. Doctor who did it, specialized in feet (don't know the nerd name for it), but he said it was the most cartilage he had seen missing. He assumes it must have been a specific incident (I didn't have one for him, other than its hurt since freshman year of HS), that got worse and worse, from other incidents.
Didn't get the ok to do anything but walk till a year after the surgery (it was new at the time I got it), and it hasn't felt the same since.
Just before I turned 30 I got sober, started working out, and went on hikes regularly. Even started doing hikes with weights in my backpack (10-20 lbs. If you try this be careful, and keep your back straight the whole time). When I started skating again, it felt better than when I was skating again around 24-26 (I was riding my bike often around that time).
I don't have any science behind it, but not drinking, and the hiking I felt helped my knee the most. It will probably never be normal again ( I swear I feel the foreign cartilage when its cold), but I can finally skate until I'm actually tired, and not because of swelling/tenderness/soreness/weakness.