I could easily see a family that shoots together gifting their child an AR pattern rifle after they got used to shooting mom’s or dad’s firearm. It gives them their own platform to customize and practice on, akin to a musical instrument.
It’s literally your comment.
I’m in my forties and what you’re describing doesn’t sound normal at all. I beat myself up good in my younger years with sports and still do somewhat regular cardio and weightlifting. I have a bum knee and hip problem, shoulder issues from weightlifting injuries, and my back gets stiff and sore on a good day.
None of that stops me from functionally living, and none of its anything the occasional ibuprofen or toke won’t fix in the short term. I can still exercise, do physical labor, open all the jars, and be generally active, and without pain the majority of the time.
What you’re describing sounds more like an inflammatory disease or auto-immune disorder. 110% get a second opinion from a different doc, or a third if needed.