>> What conclusion do you draw, Rouspeteur?
I think you can teach an old dog new tricks, he might just do them differently. Seriously though, I think that at a very young age the brain is still quite malleable and new pathways can be set down. My children are learning new words each day and creating new pathways. (They don't know which language they are learning, they just know which sounds mean what.) They will end up speaking without an accent in both languages as a result.
I first entered French immersion in Grade 6 (11-12 years-old) and although competent in the language I will never be mistaken for a native speaker. New pathways can be created but they will never be as efficient as the ones created in infancy. I would liken it to people who are able to learn how to speak again after a stroke.
Rouspeteur
