From the article:
Studies show that brains reach 93% of their maximum size when they are only 6 years old. Bigger brains have more neurons, as well as more connections between them. Genetics plays a role, but so do external factors such as nutrition, central nervous system infections and brain injury early in life. The upshot is that Alzheimer's prevention efforts should be geared toward the preschool set, the German researchers suggested.
Article: Big heads help patients withstand Alzheimer's disease
Study abstract: Head circumferernce, atrophy and cognition: Implications for brain reserve in Alzheimer's disease