Yep - we just buried one of my best friends (who played on the offensive line in college with me) as he passed away from dementia in his 40’s. There is no way to prove that this was caused by football...but there’s not a whole bunch of people being diagnosed with dementia in their 30’s.
I would be very dubious of any connection between head hits, dementia and earl onset in an offensive lineman. I am sorry your friend passed so early but I also played college football as a running back. Took a lot of hits for many years and delivered a lot as well. 63 now and IQ same as it was the day I enlisted in the Army.
I am thinking that certain percent of the population is susceptible to dementia due to heritability and concussions can accelerate the progress but if you live long enough your neuronal network is most likely gonna go down for good.