It's important to remember how much individual variation exists within the human race. I believe head trauma is an important risk factor for dementia, but there are many football players who make it to a good old age with their cognitive abilities intact. We don't know yet why some people can tolerate multiple hits to the head, while for others a similar amount of brain trauma contributes to rapid deterioration.

The bottom line (or it should be the top line) is that it makes sense to take care of your body and your brain, both so that you can enjoy life more in the moment, but because it increases the likelihood of enjoying it in the long run.