I haven't read the whole study either but it does admit to having some selection bias, most importantly that these are all deceased players (so mostly older) and their families volunteered into the study based on them having symptoms associated with brain problems, and people who had no symptoms were likely not included in the study. However the whole point of the study was to determine how the age at which one starts tackle football relates to the onset of symptoms, and a clear link was established. The earlier the player started playing tackle football, the earlier they started having symptoms.
That also introduces the complicating factors of insufficient care & treatment based on outmoded standards, inferior protective equipment, likelihood they played while concussed, etc.
So with that in mind, when discussing a disorder/disease that's cumulative when not addressed, and given they were likely in several at-risk categories (internal and external), I would expect to find a direct relationship between damage and duration of exposure.
I would also guess many were wearing helmets from the 50s and 60s when they started, as even in the 70s kids leagues were probably still using old equipment and not NFL standard. Those old helmets were just thin plastic shells with a few foam bits in them at the ears and crown (or maybe internal straps) to hold them on against a chin strap. Hell, even many pros still used the older helmets in the 70s.
