I think there is some partial credibility. There's probably an education effect. Many people who are bilingual may have a different educational level.
But there is also a possible cognitive stimulation level. As people switch back and forth between languages, they're cognitively stimulating themselves and using different parts of the brain and using the parts of the brain that are required to change association. So I think there is some validity to them.
Certainly, being a Canadian, I think it's a good idea to be bilingual, and I recommend it as a health care measure.