Our oldest son is now seven and a half and now fully understands I speak differently than most of the other people he knows. Both my husband and I tend to mimic the way other people speak, so this is a genetic trait our son seems to be manifesting, too.
Our son been casually working on his American accent so I taught him a secret I once picked up from an actor in Los Angeles.
The distinction between the British, American, and Australian accents isn't just that certain letters and combinations are spoken differently by moving your lips or forming the sound in your mouth, depending on the accent. The secret is that the sound starts in a different part of the mouth, depending on the accent.
American accent sounds are started primarily in the top of the throat and the very back of the mouth. If you're American, say a sentence now and you'll see what I mean.
But if you're British, your accent sounds are started much further forward in the mouth, away from the back of the throat entirely.
The Australian accent begins about half way between the two — towards the back of the mouth but not fully into the throat, with a few exceptions.
Of course, that's not all. The R sound in American is much different than Britain or Australia. Still, where that R sound begins is important. If you're trying to speak like an American, you need to begin it further back in your mouth and into your throat than sounds in British or Australian accents.
Of course, there are wide variations in American speak. New Englanders, for instance, don't use so-called rhotic Rs. My American accent seems more pronounced to Aussies because it's the Midwestern version, which curiously in America is considered a "non accent."
Mimicry is fun and accents are a big part of it. My brother and I were always doing them and now I can do it with both my husband and, increasingly, my son.