The trouble with doctors declaring that PMR does not occur in people under 50 is that it becomes a self-fulfilling prophesy. Since the belief is that it doesn't occur in people under 50, doctors are instructed to diagnose something else, just because of the person's age. Then when research on the occurrence of PMR is done, it appears that nobody under 50 gets PMR, because there are so few diagnoses of people under 50 getting it. So the likely fiction is reinforced.