Although it does require extra cognitive work and attention to be bilingual, bilingualism builds more gray matter in the language parts of the brain...
and it attenuates age-related declines in working memory and cognitive control!
Take-home Message: The more you use, the less you lose.
Bilingualism Prevents Cognitive Decline
Eide Neurolearning Blog: The Bilingual Brain
More Gray Matter with Bilingualism