I had the same problem, and am coincidentally, 29. I visited a physical therapist, who was able to diagnose that I have a combination of (a) New Bike Legs and (b) slightly off-set hip joints, causing my feet to role, and hence, knees to bend.
She said that I would need to (a) strengthen the knee and (b) stretch the ligaments, (c) get a proper bike fit. In addition, I needed to lay off the hills and stay in the flats, progressively increasing the gear ratio to increase the tension. 5 months later, I was big ringing it up the Front Range.
I would have a medical or physical therapist review your bio mechanics just to be sure that you may not need some sort of orthotics. The reason is that can sometimes fix a problem rather quickly and avoid other unnecessary rehabbing.