Can repetitive sports-related head injuries make a person senile many years later? Would repetitive football injuries to the cranium show up decades later, causing symptoms resembling mild retardation, OCD, etc.? What is the best way to determine this in terms of imaging, testing, etc.?
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 '