Knee injury in high school. Is there something to do at 40?

Sure. See an orthopedist and get properly evaluated by xrays and physical exam. If necessary an MRI may be ordered. Many problems can be coorected or improved such as ligament injuries and meniscal tears. However, arthritis cannot be removed. Once present it tends to progress but proper intervention can help slow the progression.
Yes. Get checked by a sports medicine orthopedic knee specialist, to see if arthritis has set into the knee, or what can be done about it.
Hard to say. Not all knee injuries are the same. Some will resolve without many long-term effects while others won't. What is clear, is if your knee is painful or it limits your recreational or occupational activities, then you should see your local orthopedist.

Related Questions

I had a knee injury in high-school. I tore about seven ligments in knee. Sometimes it feels like my knee is giving. What can I do to strengtligimennt?

Get evaluated. See an orthopaedic surgeon and have your knee examined for a proper current diagnosis. Physical therapy may be of benefit. Read more...
Ligaments. If you had several ligaments torn in your knee and did not have surgery it is unlikely that you will regain stability of the knee. You can try to strengthen the muscles about the knee with a weight training program but this will provide stability. Read more...