I had a boss who came to the US from Germany, when he was twelve. He had no German accent that I could notice.
This is me. I grew up on the East Coast but now live near San Francisco. No trace of East Coast accent is left. But speaking at length with someone with an accent that is very distinct from my own, I will start picking it up and have to consciously make myself stop.
OTOH, my Grandmother, born & raised in Oklahoma, then lived 30 years in the northeast, still sounded like she’d just stepped off the plane from Tulsa.
Speaking is a lot simpler than algebraic geometry. Even very very stupid people can speak. They do it all the time. It really isn’t hard to lose an accent. I mentioned actors not because they’re highly trained professionals but because so many of them aren’t.
If you’re not motivated to do it, fine, but I am certain that if you did feel enough motivation, you could train yourself to speak BBC English. The fact that many people don’t lose their accents doesn’t mean they can’t, it just means they don’t need to.
The only answer is, it depends. On the person, on how much they want to try and natural talent. I knew a man who claimed his family moved to the US when he was ten and he had an awful French accent. A man I currently know moved here from Paris when he was 16. Since this is Montreal, he could have just gone on speaking French, but he wanted to speak English without a French accent. He succeeded. His wife told me that he would walk down the street as a teenager, saying “the, the, the,…” over and over until he eventually learned to say the “th” perfectly (probably the hardest sound for a French speaker to master). My wife had a college friend who was 14 when her family came to the US and had a rather mild accent, there, but not strong. Her 12 year old sister has no accent while her 18 year old sister has a thick accent despite getting a PhD in English.
Then there was Peter Sellers who had a perfect midwest US accent in Dr. Strangelove.
Oh yes, let me mention Richard Feynmann. He was from Brooklyn but spent most of his adult life in California (at least after leaving Arizona working on the bomb). He lectured using a strong Brooklyn accent. Until he came to the question period. His answers were unrehearsed and most of the Brooklyn accent just disappeared. My wife (raised in Brooklyn till age 11) can put on a Brooklyn accent but rarely does it.
Certainly into the 20s. There’s Joe Namath, whose accent is part Pennsylvanian, part southern (from four years in Alabama) and part New York (though the New York is predominating).
Yes, and the vast majority of them are hopeless at impersonating accents. Given that they have a professional interest in changing accents and you claim the task is trivial for anybody who would only apply themselves, why is this so?
I don’t mean to be rude, but … you’re wrong. If you’re a neurolinguist and have some data backing up your claims that losing an accent is “easy” and can be done by anybody if only they apply themselves, then go ahead and post it. Otherwise you’re just some random schlub who’s extrapolating from their linguistic flexibility into a universal truth.
I briefly dated a guy who was born and raised in Latvia, and moved to the USA at the age of 14. He learned English with a perfect Northeastern accent; you could not tell that he was not a native speaker, nor that he had learned it so late. We were in the odd position that when we’d go out together, people would assume I (native English speaker, from the Deep South) was the foreigner and that he was the American!
As a teacher in a school with a large ESL population, I have often been amazed at how dramatically my ESL kids’ accents change after a year in college.
I think accent is really totally separate from the other aspects of language acquisition, and shouldn’t be compared. I’ve had some kids with virtually no accent but who still had very weak English skills: what they knew, they could say clearly, but their grammar and vocabulary, speaking, listening, and writing, were all poor. I’ve also had some extremely fluent kids who had accents so strong it was sometimes hard to follow them–but they could read, discuss, and write about any piece of English literature you put in front of them in an academic register.
I was born elsewhere, in a Western state, and we landed in West Texas, the South Plains, about a month before my 6th birthday. We’d lived in Arkansas from when I was 2 until the Texas move, so I guess I’d expect to have some sort of Southern accent.
Now that I think of it too, my father must have picked up a little Texas accent himself, as I recall his California relatives poking a little light-hearted fun at it one time.
Seems to vary a lot from one person to another. I grew up in the northeast and moved to the southeast at age 20. I’m now 38. I’ve been told that I’ve picked up some southern speech mannerisms but not the accent.
It seems to me that it is easy to acquire about 80% of an accent, but there’s like 20% that you don’t get, and that’s enough for you to sound off. Usually, you sound off to native speakers, but okay to people of another accent. (The case of Hugh Laurie’s American accent is curious because seems to be the opposite: Brits don’t think he sounds right, but Americans usually can’t tell.)
I’m not a speech therapist, but I used to work with one. I had a little training in linguistics, enough to assess children with speech issues and work out which ones “talked funny” because their parents were idiots and thought it was cute and which had genuine problems. 99% of the time, the parents were idiots.
It’s been over a decade since I had that job, but what it taught me is that almost everyone is able to make almost any speech sound if you show them how it’s done. Among the children I worked with were some recent arrivals in the country, so different accents don’t seem to make a difference.
So, while I’m certainly not an expert, I’m also not “some random schlub” either. Making the sounds that make up speech is so easy almost any child can pick it up instantly if they’re shown the correct way to do it. After that, it’s just practice, and practice is where most people fall down. People are usually pretty lazy with speech, even more so than with writing. If they can make themselves understood, that’s enough for most people.
The speech therapist I worked with achieved remarkable results very quickly, but only with children that understood it was important. The ones who didn’t care and didn’t practice didn’t improve. I personally saw the difference that motivation makes to speech clarity. It makes all the difference. I didn’t see what happened to the handful that had genuine difficulties in the brain or in the mouth parts, because that happened elsewhere, but I did see children go from incomprehensible to a non-family member to speaking clearly in a matter of months.
It really depends on the person- my uncle emigrated from the UK to Australia with his 3 kids, age, I think, 8, 12 and 13, at some point in the 1970s. When I went to visit a few years ago, the one who left age 8 still had a tiny trace of a UK accent, the one who had been 12 still sounded almost totally English, and the eldest one was the only one who had lost all hint, and sounded (both to me and Aussies) completely Australian.
His two later kids, both born over there, had a few words they sounded very English on.
My uncle on the other hand has one of the strongest Lancashire accents I’ve heard in years, much more so than anyone in the family who stayed living in England. 
Was quite funny going on one of the trams with him, and having the ticket collector carefully explaining where we should get off, being helpful to the tourists, and watching his reaction when my uncle, in totally broad Lancashire, cheerfully told him he’d driven the trams in that town for over 20 years…
Hi! I was wondering the same thing as you which is how I came across this feed. I just wanted to talk about my own experience. I was born in NoVa (Northern Virginia) and I lived there until I was 9. The area of Virginia I grew up in was, what my friends and I joke about, an area with NO accent. Everyone said every word as plainly as you can say them. When I was 9, my family and I moved to South Carolina. I’m now 16 almost 17 and I noticed when talking to my friends a few years ago that I’ve picked up a southern accent. So I guess I picked it up maybe around age 10 or 11. I don’t know if I fall under that category of taking on the accents of those around me or not. I just find it interesting and if you find an answer you really like, let me know 
In other words, you only dealt with children, the group that everyone in the thread agrees can pick up new accents. And not even as an expert. You merely worked with an expert, who you aren’t even citing.
The actual data I’ve seen is that we lose our accent plasticity in our mid to late-twenties. We literally lose the ability to use certain muscle combinations if we aren’t used to doing so. It’s no different than how difficult it is to learn an instrument past that age or even learn to be an athlete.
I pick up accents like a sponge and need to work to eliminate them. I have, however, had years of theatre and acting training, so that may be why.
Many of my relatives from my Mother’s side grew up in either Brooklyn or Nassau County LI, and moved to Myrtle Beach, SC. Those that have been there for 20 years or so sound more Southern than NY’ish, although my understanding is that the two accents have more in common than most people think (due to the fact that they’re both non rhotic), so maybe the distinction isn’t that noteworthy.
I moved from West Virginia to Columbus, Ohio when I was 11. When I first moved here, people always asked me if I was from the South. Nowadays I sound like anyone else from Columbus–practically like a newscaster.
I have a normal American accent, without any particularly recognisable regional characteristics. My younger brother and older sister both have much softer AUS accents.
Both retained strong American accents until they returned to the USA for several years after completing university education. Going to AUS the SECOND time, after working in the USA, they lost most of their American accents.
I can not hear American or Australian accents. Both accents are in my normal range of what I expect English to sound like. I hypothosize that on returning to AUS the second time, my sibs were able to hear the AUS accent and adjust their own.
I don’t buy it. I’ve known far too many people who came to the US after the age of 8-10 who, within just a few short years, were speaking with a perfect US Midwest accent.
My guess would be puberty to early adulthood.