Don't ignore the pain ...
I'm 42, been lifting relatively heavy since I was 16, including heavy squats and presses up to 3x a week in my early 20's. No real pain until I played softball for 2 or 3 seasons, then squatting bothered me, so I either did them light, or just presses, but pain was there almost always, kind of a nagging pain. Occaisionally I'd feel okay and do some light squats, up to 315 for 12 - 15 reps, but there are times I'd go 6 months unable to do anything but light press and stairstepper.
Recently, I've been able to get back into squats, not really heavy, but as heavy as I've been in 10 years with little / no pain ... 405 3 sets x 8 reps (not failure). I've slipped a disc in my back before, so I'm reluctant to go to failure.
Well, on Feb 25 2010, I went to do squats in the rack at the gym, warmed up on the bike 10 mins, 3x light extensions and curls, 2 x 135 15 reps, 2 x 225 12 (no pain), 1 x 315 12 reps easy no pain, 405 1 x 8 reps easy no pain, then on 2nd set of 405, on the 4th rep or so, on the way up my right knee "snapped", sensation of breaking bone, then left knee did the same. I had the safety rack in place just beow paralell, so I only fell about 3 inches, but I had no control over my lower legs, could not raise my foot with the extension movement at all. Went to the ER via ambulance, had a bilateral quadricep tendon rupture, torn completely off the bone on both legs, and surgery the next day to repair. Now I'm in double full length casts for 6 more weeks, then PT. Doc said there was evidence of prior partial separation from the patellar, which was where my pain came from prior. So, don't just mask the pain and keep moving, try to get it assessed.
I was qualified t go to IFBB Nationals in the Masters and Open this summer, but that is down the drain now, and I have a 12 - 15" scare on each leg to show for it. BE CAREFUL FOLKS !