According to a report published last fall by the European Commission entitled "The Contribution of Multilingualism to Creativity," one of the major benefits of learning another language is reducing age-related mental diminishment.
Knowing another language seems to slow down the rate of decline of certain cognitive pathologies. Being multilingual helps resist neuropathological damage by 2-4 years. In addition, multilingualism is linked to a slowdown in age related issues such as dementia.
The bonus...learning a foreign language has a strong link to creativity, which has greater potential for cognitive health for the aging mind.
What else are you doing to slow down the aging of your brain?