Is this what happens with an untreated ACL injury?

Nearly 20 years ago, I hurt my knees (i.e., they were extremely sore) after an ill-fated attempt to ski down an icy slope. (I couldn’t control my edges, and so kept sitting down - while my ski tips were pointed in a V - to keep from flying off the trail into the woods.) Ever since then, any sort of activity that puts stress on my knees, especially the left one, leaves them sore for a while. If I do anything particularly harsh, like lots of hiking up and down slopes, I can end up with a stiff knee that I can’t bend - it feels like something is physically inside my knee, keeping it from bending. If I accidentally lean too hard and force it, I can feel a snap and a shot of pain, but then my knee can move normally again, and the pain goes away pretty quickly.

Last week I had to negotiate some uneven ice-covered ground while lugging a fairly heavy pack. My left knee was extremely unhappy after this exercise, with stiffness like before but now with a fair amount of pain whenever I put weight on that leg. I’ve rested it for a few days and popped some ibuprofen… Now it’s much improved, still a little stiff when I bend it (especially if there’s a sideways component to the motion, like putting my left ankle up on my right knee) but no pain. What IS new is what I’m hearing when I walk up and down stairs - I can only describe it as a ratcheting sound, a sort of click-click-click-click. I only hear the sound if my leg is bearing weight, but that’s the real potential problem, isn’t it?

A friend of mine suggested that I might have damaged my ACL all those years ago, and I’m feeling the effects of accumulated damage now. I would have thought an ACL problem would have reared it’s ugly head sooner, especially given the stress I put on my knees while on the job, but maybe not… Any thoughts on what’s up with my knee?* Do ACL injuries generally need surgery to repair? What actually happens if an ACL injury is left untreated?
[sub]*I know I need to see an orthopedist. Having just started with a new insurance plan that has a large deductible, and not a lot of cash in hand, I’m not in a position to go see one right now. I’m just looking for a bit of speculation, no diagnoses, and I hold no one responsible for any advice they may care to give.[/sub]

For a torn ACL, surgery is indicated. If you’d like to go around with a knee brace, you can do that.

My orthopedist told me that the knee gets more unstable over time and you’ll end up tearing( & re-tearing) your cartilage.
I am not a doctor of course, but I have had cartiage repaired and an ACL reconstruction. It happened the same way. Skiing, a while ago. The knee came back ok, but over time the rest of the tissue could not compensate for the ACL and I kept tearing cartilage. It sounds like the locking & popping & clicking is cartilage tearing, although it could be something else.

The ACL reconstruction was the best thing I could have done! I have more range of motion in my knee, it is more stable than before, and I can do physical activities with confidence.