Studies of normal brain aging have reported shrinkage of hippocampal and frontal lobe volume in humans but not in other species. However, comparable data are not available in chimpanzees. In this study, the authors used MRI to measure the volume of the whole brain, total neocortical gray matter, total neocortical white matter, frontal lobe gray matter, frontal lobe white matter, and the hippocampus in chimpanzee brains encompassing the adult lifespan from 10 to 51 y of age. They compared these data to brain structure volumes measured in adult humans from 22 to 88 y of age. In contrast to humans, who showed a decrease in the volume of all brain structures over the lifespan, chimpanzees did not display significant age-related changes. They concluded that the increased magnitude of brain structure shrinkage in human aging is evolutionarily novel and the result of an extended lifespan.

Sherwood CC et al. Aging of the cerebral cortex differs between humans and chimpanzees.  Proc. Natl. Acad. Sci. USA 108(32):13029-13034 (2011).

http://www.pnas.org/content/108/32/13029.abstract?sid=b6ebdf52-3776-4548-9b5a-9dec6975ba25