Originally Posted by
Velo Dog
As everybody else has said, position is important. Knee pain can come from having the saddle too low, so check that.
Another thing to consider, though, is what you're doing. It's pretty common for young guys to go hard all the time--if you're riding every day, and especially if you're pushing too high a gear, your knees are likely to complain.
+1. I had some bad knee pain in early January after a long, all-day ride. After making some adjustments, I believe the problem for me was low seat position. I raised the seat a good bit (1.5 inches). I also repositioned my cleats. The suggestions of a good fitting should be your top priority. Keeping a faster cadence also helps.
Walter