You should go see an ortho and get your knee xrayed. You may have a situation where your cartilage behind your knee is deteriorating, and you will be prone to develop arthritis. It happens even when you are young and is not age related. They will probably tell you to take glucosamine and chondroitin supplements, and switch to low impact exercise, like biking or swimming. I’ve noticed the clicking in my knee since childhood. I was limited in the amount I could run in my 20’s by knee pain, but did nothing about it. I’m now in my 50’s and stairs can sometimes be problematic. Biking strengthens the muscles around your knee cap without the strain of impact you get from running. While your body is created to run and move, it is not engineered to run on asphalt. The supplements really help, btw, but take about a month before you can notice the effect.