Many times, we know of someone in the same situation, who moved to a new country with a foreign accent and somehow managed to pick up the local accent.
Maybe it's a younger sibling
Maybe it's a younger cousin
Maybe it's a nephew/niece
Have you noticed the pattern? Usually, this person is younger than you. They seem to pick up the local accent so naturally. They didn't really pay attention or put in extra effort. Why are they better able to integrate or grasp the English language?
There's a few things going on but as in most scientific research, isn't so black and white.
1. There are brain changes going on.
When you are born, your brain is like a blank page, ready for all inputs and sensitive to all stimuli. This applies to sounds too. In the beginning, babies can hear the subtle differences between sounds. However, as you age, this ability to hear the difference begins to wane. We're talking about within 6-months of being born (1) where babies start to show a preference and at 1-year when they stop responding to certain stimuli. And it just gets harder and harder the older you get - with noticeable performance differences in accent and grammar at age 7-8.
Why does this happen? Because our brains are efficient. If you haven't had to tell the different between /r/ and /l/ (like with Japanese speakers), your brain decides the extra effort/cost of being able to tell those two sounds apart is not worth keeping and lets those brain connections stop. It's the same reason you don't have a VHS player anymore - you don't use it anymore because you don't interact with VHS tapes anymore. You purge what you don't need.
2. More brain changes.
There's been a scientific debate about whether the types of memory used in language learning differs as we age. It was first thought that children learn primarily by implicit (procedural) learning processes because of how immature their explicit (declarative) memory systems are (2).
In recent years, there's been new research that shows improved performance in young adults and adults as compared with children (3) - that there may actually not be a childhood advantage (4). It reframes the challenge that older individuals have with language learning . Maybe experiential factors hinder effective use of skill learning mechanisms. It's like when you're in grade 9 and love gym class but dislike math class. It's cause you have years of experience knowing the difference between the two that you become selective. Now imagine a brand new kid going to school. It doesn't matter what subject they're learning, everything is new and therefore everything seems interesting.
The good news is that there are some situations in which adults perform better than children. In other words, if we can figure out how best to teach adults, they may be able to learn a language skill better than children. In other words, current teaching methods are not taking advantage of adult English learners' language learning skills.
So this next part is my responsibility. I need to make sure I am up-to-date on current language learning research to best offer my services.
1. Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001. The Development of Language: A Critical Period in Humans. Available from: https://www.ncbi.nlm.nih.gov/books/NBK11007/
2. DeKeyser R, Larson-Hall J (2005) What Does the Critical Period Really Mean? In: Kroll JF, De Groot AMB, editors. Handbook of Bilingualism: Psycholinguistic Approaches. Oxford: Oxford University Press. pp. 88–108.
3. Ferman S, Karni A (2010) No Childhood Advantage in the Acquisition of Skill in Using an Artificial Language Rule. PLOS ONE 5(10): e13648. https://doi.org/10.1371/journal.pone.0013648
4. Dorfberger S, Adi-Japha E, Karni A (2007) Reduced Susceptibility to Interference in the Consolidation of Motor Memory before Adolescence. Plos One. 2. (2).e240 p. Accessed February 28, 2007.