A recent study, published Jan. 2013 in   The Journal of Neuroscience,  indicates:

"While the effect has been noted before, in a new study published Tuesday in the Journal of Neuroscience, Dr. Gold and his colleagues have found that older subjects who routinely employ two languages also use less energy as they alternate between mental tasks.

The results reinforce the idea that by continually switching between languages, bilingual speakers are helping to build up their “cognitive reserve” - the synaptic stamina that can delay the onset of dementia in later years.

The researchers argue they demonstrate that bilingual speakers, on average, appear to be better at retaining cognitive control in their later years. Neuroscientists associate cognitive control with both an ability to switch smoothly between mental tasks and with keeping conflicting signals at bay when the brain is trying to stay focused."