Can repetitive sports-related head injuries make a person senile many years later?
yes, here is a nice paper which shows evidence : http://www.nature.com/srep/2013/131017/srep02972/full/srep02972.html

from the paper :
" a report commissioned by the National Football League (NFL), showed that retired players between the ages of 30–49 were 20 times more likely, at a rate of 1.9%, to receive a diagnosis of dementia, Alzheimer's disease (AD), or other memory-related impairments11. Over the age of 50, the proportion diagnosed with one of the above rose to 6.1%, compared with only 1.2% In the general population11. Overall, neurodegenerative mortality of NFL alumni is 3 times greater than that of the general population, with AD and amyotrophic lateral sclerosis (ALS) estimated at 4 times as high '