what causes it?
with me it was a combination of things. Wear and tear plus rubbish knees. I have knocked knees and have exercised quite a lot (and still do) and this seems to have done them in. I am only 32 mind, so you have to think that bad luck and genetics play a major factor.
I've had a lot of problems with OS's and PT's so can't really give you good guidance from a professional point of view about what's best. My basic rule is 'if it hurts don't do it'.
What's worked best for me is to do quad strengthening exercises every day. I also practice yoga, but with focus on anatomical alignment. I do squat very carefully but squatting puts enormous amount of pressure on meniscus so please be careful. You should never ever twist the knee (dodgy knees or not), hip, knee, foot in a straight line always.
I cannot really run - it really hurts my knees, but I think that it very much depends person to person.
Damaged meniscus cannot heal because of a lack of blood supply. Therefore, you must be aware that if you make your knees worse they will not get any better. Pain has a drip drip drip effect. Do not take chances with your knees, its a difficult lesson to accept and learn.
I think that my OS will eventually give me further treatment. Its likely that a damaged meniscus will eventually tear. But I'm waiting for as long as I can. I already had plica removal and found that operation made my knee worse. Do what's best for you.