I'm not exactly sure how accurate this is, but I remember reading an article a few years back that most people can only truly learn another language and pick up that language's native accent and still be able to speak their native language with their native accent when they move to another country before they turn 10. I think that's true because I know this American/Brazilian family and their kids speak English the way people from the USA do and they also speak Portuguese the way people from Brazil do. I guess it's really hard to truly pick up an accent even if you live in a place for so long. I haven't ever been abroad, I have never been to an English speaking country. I do, however, have a lot of friends from the US and some of these friends, we have some Brazilian friends in common who have lived in the USA and they still say my English sounds better than theirs, so I guess it really depends, it depends a lot. I mean, I think I speak okayish English, but I doubt I'd ever be able to speak the American accent nor the British/RP one. I just sound like I do and I guess that if I ever went to an English speaking country and spent some time there, maybe I'd be able to improve it a little but never to a point I sound like a native speaker.
I've seen Max Cavalera speak English, he was born and raised in Brazil and only learned English later on, he married an American woman, lives in Arizona, all his kids speak only English, not Portuguese and I've met him in person and his Portuguese sounds kinda gringo and I've seen him do interviews in English and you also can tell he has an accent, and what's more weird about that is that I think his brother Iggor Cavalera -- who has a Brazilian family, has lived in the USA but then came back to Brazil -- sounds more native-ish speaking English than his brother does even though I'm sure he didn't get to practice it as much as Max did, and not to mention that I've met Iggor too and he definitely does speak Portuguese like a Brazilian.
So, as I said before, I guess it does depend a lot. I have this friend from Norway who married an American guy and they spent three months in Brazil. She hadn't spent that much time in the USA as it was a long-distance relationship before they got married and I met them shortly after they got married and yet she sounded like an American to me, when I met her, I was surprised to learn she was from Norway. But then, upon talking to another friend of ours who was from the USA but who was not her husband, he said that she definitely did sound like a native speaker but if you paid enough attention, you could tell she was foreign but it's barely noticeable. What also amazes me about her is that she didn't speak Portuguese but I did teach her a couple of sentences and she could repeat those sentences and sound almost like a Brazilian.
I guess some people just have a gift for adapting their accents from language to language. And, yeah... Even though I love to learn languages and about languages, I'm not one of those people.