Age 5 is the optimum age for learning another language.
I have a good friend who's a gerontologist, and another who tried to learn Spanish for her grand-daughter's sake; both say that it is virtually impossible for an "old" brain to learn a new language. Also witness to this is the old lady at church who comes from Ghana, I think--she came over a couple of years ago and has nearly no English. She sure loves it on fourth Sundays, though, when the Africans sing in their own languages!