One of the many problems in science is the mistaken belief that correlation = causation.
That is, people the fact that more men then women are CEO's does not indicate men are better at business, instead it indicates that men are favored by the people that select CEOs.
Here are several other possible explanations for the survey results:
1) Intelligent people are both more likely to learn multiple languages and more likely to age better because they take better care of their brain, or it could be that the same trait that makes them smarter also protects their brain. I.E. Smarter people get less brain deterioration and smarter people learn more languages.
2) The brain's very first bits of degradation occurs at a very young age (before 30) and that same degradation makes learning new languages unpleasant and/or more difficult. I.E. later brain deterioration prevents you from learning languages at an early age.
I am not saying these are true, I am saying the idea deciding to learn a language will protect your brain is NOT supported by the data, at least not anymore than my 2 suggestions.