Sub-concussive impacts (SCI's) have been studied extensively. There are athletes who have never suffered a concussion who have played sports where sub-concussive impacts are common who have developed severe symptoms of PSC, some in their early 20's.

A sub-concussive impact is any impact to the head that does not result in an immediate or even delayed feeling of being concussed.

Football linemen and hockey players are the usual sufferers of sub-concussive impacts. The average football player who plays positions where SCI are common receive between 800 and 1000 per season. They are cumulative.

The article I posted shows what can happen from SCI's

It is often the most damaging head injury because there is not time taken off from play to recover. I can tell you more but the simple truth is any hit to the head cause a risk of damage.