A long-running study involving 1,157 healthy older adults (65+) who were scored on a 5-point scale according to how often they participated in mental activities such as listening to the radio, watching television, reading, playing games and going to a museum, has found that this score is correlated to the rate of cognitive decline in later years.
Some 5 ½ years after this initial evaluation, 395 (34%) were found to have mild cognitive impairment and 148 (13%) to have Alzheimer’s. Participants were then tested at 3-yearly intervals for the next 6 years. The rate of cognitive decline in those without cognitive impairment was reduced by 52% for each point on the cognitive activity scale, but for those with Alzheimer's disease, the average rate of decline per year increased by 42% for each point on the cognitive activity scale. Rate of decline was unrelated to earlier cognitive activity in those with MCI (presumably they were at the balance point).
This is not terribly surprising when you think of it, if you assume that the benefit of mental stimulation is to improve your brain function so that it can better cope with the damage happening to it. But eventually it reaches the point where it can no longer compensate for that damage because it is so overwhelming.
 . Cognitive activity and the cognitive morbidity of Alzheimer disease. Neurology [Internet]. 2010 ;75(11):990 - 996. Available from: http://www.neurology.org/cgi/content/abstract/75/11/990