I'm leaving this article up for awhile, to give plenty of people a chance to find it.
This article is quite technical and difficult to read unless you are used to reading research articles. The gist of it is that marijuana use helps prevent formation of plaques in the brain, which are characteristic of Alzheimer's disease. If I can figure out how to post the link, I will but here is the title:
A Molecular Link Between the Active Component of Marijuana and Alzheimer's Disease Pathology
Authors: Lisa M. Eubanks, Claude J. Rogers, [...], and Kim D. Janda