13 years if you speak 4 languages, 6 years for two languages:

> The study found that those who spoke two languages had brains that appeared around six years younger than those who spoke only one language. People who spoke three languages had brains that appeared around seven years younger, and for those who spoke four languages, their brains appeared about 13 years younger.

Learning is the brains way of exercising, though I believe you genuinely need to want to learn something which helps in the slowing of ageing part