I have a somewhat bad knee from a past accident, so my knee starts to hurt after skiing all day. Using a knee brace helps. But I would think skiing strengthens your knees, not hurts them. It's a great workout.
It is advisable to do a variety of exercises to strengthen the quadriceps muscle at the front of the knee. Squats are one exercise that is particularly helpful. Strong muscles will help with the pounding that the knees take and they take a lot.
Skiing may be hard on the knees if you do not warm up sufficiently before ypu start skiing. I also would advise you not to push yourself too hard and ski on tough terrain. You want to enjoy skiing for many years so you shoul avoid hurting yourself.
