Im 29, had 4 arthroscopies…2 on both knees.
The right knee is now stable and since 97 hasnt really given me problems.
The left knee gave me problems until hopefully now. I have just had an op to 'straighten' the leg, which was, in their view at fault. I didnt have a well aligned left leg, and so the forces that i put through the knee were incredible as the line of force didnt actually pass through the knee itself, but to the side. In realinging the bones, hopefully the forces will now pass through the centre of the knee and be supported by a knee that is stable enough.
The risk with arthroscopies, and hence the reason that many people have multiple and not just one, is that in trimming the cartilage they leave a sharp edge which can then quite easily catch and re-tear.
Get a second opinion regarding your alignment, as this was not seen by any doc in the UK, only here in Switzerland by a renowned sports doctor. There may be a reason why you have both knees like this. Just a thought, not saying it will be the case.
edit: oh and re: sports.. the only sports i have stopped are badminton & tennis. Due to my long history of knee probs i have never tried snowboarding. I can never try it as its seen as one of the worst for damaged knees.