Knee discomfort due to use and knee injury may be very similar. At your age, the accumulated wear and tear of a lifetime of sports may give you symptoms that are very similar to the effect of a single, traumatic injury.
What you probably have is a little torn cartilege and also a little arthritis in your knee joint. It will take an MRI to confirm the condition of your knee joint.
If this is the diagnosis, then you will probably be advised to just continue on until you have too much difficulty, and then you get arthroscopic surgery to vacuum out the loose bits and pieces.
You should do low-impact exercises to strengthen the muscles around the knee, and possibly take glucosamine tablets, although there is little evidence that this will actually regenerate any new cartilege.
Also, your skiing technique may affect your knees. Stance and balance is important - stay centred for and aft on your skis - if you sit back while skiing, this could put extra stain on your knee joints.