...it’s very important to use your brain, to keep challenging your mind, but all mental activities may not be equal. We’re seeing some evidence that a social component may be crucial... The evidence suggests that people who spend long stretches of their days, three hours and more, engrossed in some mental activities like cards may be at reduced risk of developing dementia. Researchers are trying to tease apart cause from effect: Are they active because they are sharp, or sharp because they are active?
.. So far, scientists here have found little evidence that diet or exercise affects the risk of dementia in people over 90. But some researchers argue that mental engagement — doing crossword puzzles, reading books — may delay the arrival of symptoms. And social connections, including interaction with friends, may be very important, some suspect. In isolation, a healthy human mind can go blank and quickly become disoriented...
“There is quite a bit of evidence now suggesting that the more people you have contact with, in your own home or outside, the better you do” mentally and physically, Dr. Kawas said. “Interacting with people regularly, even strangers, uses easily as much brain power as doing puzzles, and it wouldn’t surprise me if this is what it’s all about.” Link
It's fairly intuitive that people are happier and more content when they have friends and family surrounding them. However, prior to reading this article I would have considered the memory benefit of puzzle solving, crosswords etc... to be fairly independent of the social component for avoiding dementia. I also would have thought diet would play a greater role.
On the other hand, loss of social interaction probably reduces mental agility across all age groups regardless of dementia. Because social isolation in non-dementia adults is reversible, it is probably ignored when considering poor cognitive performance. However, if life decisions such as moving cities, having children etc... are a serious handicap to cognition, then perhaps we should be even more concerned about the loss of community and fragmentation in our society?
Also, the benefits of socializing might also justify a moderate partying lifestyle amongst undergraduate students*.
* Although this is potentially offset by binge drinking and drug use.