Well, she is technically an English speaker, if you can call Scottish English, but I think that Scottish actress Kelly Macdonald really nailed a sort of southern American accent in No Country for old Men.
No Country for Old Men (9/11) Movie CLIP - You Don’t Have to Do This (2007) HD - YouTube
Cite? My personal experience is that six and seven and even eight doesn’t present a barrier to learning a second language accent-free. Not always, of course, and often not as easily, but I grew up with a lot of immigrants in grade school, and some of them caught on really well through at least third grade.
If you poke around google looking for cites, you’ll find numbers all over the place. Most commonly you’ll find the cutoff age is given as somewhere between 6 and 9. People vary, so the language cutoff age presumably varies as well.
In my own experience, my father spoke Greek until he went to kindergarten and he did not have an accent at all. My grandmother, on the other hand, had one of the thickest Greek accents I have ever heard when speaking English.
Researchers have figured out that after the cutoff age (let’s call it 8-ish) you actually learn language differently in your brain, and after this age you have much more difficulty in both learning a new language and also in hearing that language’s phonemes. For example, if a child grew up speaking Japanese but learned English before age 8, they would learn it easily and would be able to speak it without an accent. But if they started learning English after age 8, they would have more difficulty learning it and also would have great difficulty distinguishing between certain sounds. The English L and R sounds for example don’t have two distinct equivalent sounds in Japanese. Instead, Japanese has a single sound that is kind of a cross between the two. Children before age 8 can hear the difference. Children after age 8 lose that ability.
Mid-teens, as specified in the OP, is well beyond the cutoff age, so anyone who has managed to learn English after that has not only put in a lot more effort to learn the language, but has also gone through a great deal of effort to learn and speak the distinct sounds of English as well in order to lose the foreign accent.
So Mila Kunis (mentioned upthread) probably had a fairly easy time of it since she learned English at age 7. Alona Tal (mentioned in the next post) learned English in her 20s, and so deserves a lot more credit. She had a much more difficult task.
One of the funnier experiences I had, the other way around, was when I lived in France and acquired a pretty good fluency in the language. There were a bunch of British ex-pats I sometimes hung around with, one of whom had a daughter, age 4 or so, whose French was impeccable (for 4, of course), totally native and fluent, much more so than mine. Her parents, however, didn’t even try speaking French. They were from the “Shout at 'em in English, mate, and they’ll understand you” school of ex-pats, really comically boorish, and the daughter regarded me with a very queer suspicion. She refused to speak French with a non-native, and with other Brits and Americans and Canadians, she would pretend not to understand French at all. My French, though, had gotten pretty good, and when I spoke to her, she was wary of me. I could see her trying to decide if I could be answered in French or if i was another complete Anglophony.
And some are either lousy at it or can’t be bothered - like Sean Connery, who was less than convincing as an American or Russian.
Another one from the UK: a leading journalist/presenter on TV is Matt Frei, who was born in Germany, only came to the UK at the age of 10, went to British schools, married and settled here, and you wouldn’t know he wasn’t born and brought up here all his life.
I just finished watching The Night of the Iguana for the first time and thought of this thread.
Deborah Kerr made not even the slightest effort towards speaking with an American accent. It was especially noticeable since her character actually says that she was born and bred on Nantucket. The actor playing her grandfather (Cyril Delevanti) was UK-born and bred, but he sounded American (although he had been in the US for decades before). Kerr didn’t even attempt a broad mid-Atlantic Yankee accent a la Katharine Hepburn. It was jarring.
OTOH, I was still a kid when the movie came out, and I had a vague memory of it being shocking and controversial. It was actually rather sweet, I thought. Tangentially, Ava Gardner was gorgeous. I never was that impressed with her looks, but mostly I saw her with over-the-top 50s and 60s makeup. Her makeup was much more subtle here, and she was the better for it.