Getting old sucks...true that.
If I'd stumble on a tree root that would cause my foot to get torqued to the left or right side, I'd have a bad stabbing pain, that felt like it was on the inside, but left side of my right knee. The Doc said that the tear is basically a flap of material that just gets cut away. There's no way to stitch it, so once it's removed the pain goes away and it heals up.
My original right knee injury was 39 years ago, in '84. It involved my knee cap not tracking straight up and down, as I'd extend my foot. The knee cap would partially drop off to the right side of the joint....which is actually a partial dislocation.
It didn't take much of a twist or turn to cause the entire knee to dislocate, which was excruciating pain. I reset it myself too many times and at age 26, decided to have the reconstruction surgery. This had me off of work for 3 months....
BUT this was back in the dark ages of 1984 and even this surgery has greatly improved to less than 4 weeks. Rehab was an absolute bitch!
After it was all said and done, I was faster with that leg, but it took me the better part of 8 to 9 months to get the power back into it.
I was in the height of my kickboxing days, from age 19 to 36 and I was driven to get back in the ring, which helped with my recovery.
If you're saying your knee "just goes out for no reason", I'd definitely have it checked out, because it ain't gonna get any better on its' own. The younger you are when you get it repaired, the quicker you'll recover and the easier the rehab.
Good luck, brother!!!