I think that Madonna's accent is probably a little affected rather than effected. I know several people who can "swap" their accents at will to impress the people they talk to. Most are women who want to dispose of their regional accents to attract a better class of person or to be accepted in social circles.
Kids tend to pick up accents quicker as they learn new words and pronounce them in the regional dialect.
However, watching a recent programme called "NYPD Green" about Irish emigrants who work for the NY Police department it's interesting to hear their Irish accents interpersed with Americanisms that they could only have picked up from work parlance. Words like "suspect", "vehicle", "side-arm", "perpetrator" and "weapon" (which sounds like weppin' when said with a Noo Yawk accent). Clearly the damage has been done at a young age and only new, unfamiliar words will carry a new accent.
