My boys are interesting when it comes to accents. The eldest (my stepson) moved here with my ex and I in 1999, lived in California for 5 years before we moved to Georgia. He's now 13. The youngest was born in California and lived there for 3 years before moving to here. He's now 6.
Both of them have English accents at home.
The eldest will have a pronounced Californian-American accent when he's talking to his peer group, but he can turn straight around and talk to me with an English accent. I don't think he even knows he's doing it. He has no hint of a Southern accent at all.
The youngest has a English accent the majority of the time, even with other American children. He does use American words, such as 't-made-o' (as opposed to t-mart-o) and will occasionally use an American accent like his bigger brother. He can put on a wicked southern drawl if he wants too, but it's usually when he's joking about and talking about huntin' and Wal-Mart (honestly).

It seems as though their accent stems more from the family unit than their surroundings, which may explain the post on the bottom of the previous page to some degree.

you see a similar phenomenon beyond accents and into languages. There are lots of ABCs (American Born Chinese) that will speak English to their parents, and the parents will speak back in Cantonese or Mandarin. And then sometimes get into a weird mix of Chinglish.

In America, the Army G.I. Bill has indirectly had a large influence on language, as it opened universities to different classes of people other than the wealthy. In literature, this brought a much different style of writing, not to mention themes and subject matter few had previously been exposed to. Linguistically, the same sort of style changes have occurred as regional and class-oriented speech was introduced by more rurally raised students and professors.

I've always thought that adopting the accent of the place you move to has more to do with the person's personality than anything else. My sister grew up, went to high school, college and worked in the midwest before getting married and moving to Georgia. She was a teacher and spoke quite well before she moved. Within a few years she had a perfect Georgian accent. It fades noticably if she comes back home for a couple of weeks. If I moved south I don't think I could change my accent if I wanted to.