My friends 8 year-old son has the habit of using the accent of whichever parent he is talking to.
His father is from the middle of the country and pronounces things like "grass" and "bath" with a short 'a' while his mum is a southerner who pronounces things properly and uses the long "grarss" and barth" form!

Its quite funny to hear him change halfway through a sentence when he starts talking to the other parent!
I also pick up accents quite quickly apparently. I went away for a few months and when I came back, my mother asked why I was talking with a Manchester accent. I didn't realise I was!
Glen