Researchers from the Department of Psychiatry at the University of California at Davis in Sacramento, California, investigated if and how meditation might preserve cognition and prevent dementia.
Previous studies have indicated that meditation affects multiple pathways that play a role in brain aging and mental fitness. For example, meditation may reduce stress-induced cortisol secretion and this could have neuro-protective effects by elevating levels of brain derived neurotrophic factor (BDNF).
Meditation may also potentially have beneficial effects on lipid profiles and lower oxidative stress, either of which could reduce the risk for cerebro-vascular disease and age-related neuro-degeneration.

Further, meditation may potentially strengthen neuronal circuits and enhance cognitive reserve capacity.
In one cross-sectional study, meditation practitioners were found to have a lower age-related decline in thickness of specific cortical regions.  But findinga are still inconsistent and preliminary, and represent a heterogeneous group of practices that need to be sorted out.
Key future challenges include the isolation of a potential common element in the different meditation modalities, replication of existing findings in larger randomized trials, determining the correct "dose," studying whether findings from expert practitioners are generalizable to a wider population, and better control of the confounding genetic, dietary and lifestyle influences.

Citation:  Xiong GL, Doraiswamy PM.  Does meditation enhance cognition and brain plasticity? Annals of the New York Academy of Science. 2009 Aug; 1172: pages 63-69.