Studies linking head trauma with increased risk and earlier age of onset for Alzheimer's disease have yielded contradictory results. Now a population-based study involving 448 healthy older adults (70+) and 141 seniors with mild cognitive impairment has found that a history of head trauma was associated with higher levels of amyloid-beta plaques (a marker for Alzheimer’s) in those with MCI, but not in the cognitively normal. Similar rates of self-reported head trauma were found in the two groups (17% and 18%, respectively).

http://www.eurekalert.org/pub_releases/2013-12/aaon-acr122013.php

[3591] Mielke, M. M., Savica R., Wiste H. J., Weigand S. D., Vemuri P., Knopman D. S., et al.
(2014).  Head trauma and in vivo measures of amyloid and neurodegeneration in a population-based study.
Neurology. 82(1), 70 - 76.