Thanks for that. I would imagine a very broad spectrum of people become bilingual: some from birth, some because of emigration, voluntary or enforced, etc. so I don't think bilinqualists comprise a group as such. As you say, it's difficult to sort the variables. I seem to remember some time ago a discussion about this on tv (UK), discussing whether mental activity helped to protect the mind and which featured a theoretical physicist who was still working well into old age.
I don't fancy the type of caloric restriction that people practice in order to live longer, but I'll keep on with the studies! How about you?
(goes off back to the language tapes)