I don't think the age limitation for PMR is set in stone. It is possible to be diagnosed with PMR before the age of 50 but it is rare. I was 52 when I was diagnosed so I was young for PMR too.

I can relate to you because my problems started when I was young, There are other conditions that are similar to PMR. Have your doctors ever considered Juvenile Spondyloarthritis? There are several types of spondyloarthritis and they are very similar to PMR.
https://spondylitis.org/about-spondylitis/overview-of-spondyloarthritis/juvenile-spondyloarthritis/
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The thing that seems unfair to me is that having spondyloarthritis at a young age doesn't prevent you from developing PMR when you are older. Some studies suggest that having one autoimmune disorder increases the risk of developing more than one. There are other risk factors that are outlined in the following link.
https://www.autoimmuneinstitute.org/7-ad-risk-factors/