Linguists are still divided on this topic, called the “Critical Period” hypothesis - the question of whether there is a “Critical Period” during childhood when children naturally acquire language better than adults.
The data in favor cited in pop articles often comes from “feral children” like Genie, but as Quetzalcutlass@lemmy.world mentioned, how much of this inability is due to natural brain development and how much is due to years of unimaginable trauma is hard to know.
Other research has cited brain plasticity differences and brain matter changes that occur during puberty that seems like it may be linked to language acquisition.
Again, however, the counterpoint of “It takes ten-ish years of pure immersion for children to learn a language, and how many adults actually do that” is pretty frequent.
I’m still undecided about what I think - maybe something in the middle, like “humans do lose some neuroplasticity during puberty that may inhibit language acquisition a bit, but adults acquiring native-like fluency is still possible with enough immersion”.
There was an old study showing that London taxi drivers develop enlarged hippocampi, the part of the brain used for navigation, to deal with the labyrinthian London streets. The growth continued over several years even in mature adults as they used those navigation and memorization abilities. I’d like to see a study of the brain of an adult prospective language learner over a long period to see if any similar plasticity exists for the brain’s language centers.
(I’ll admit I’m horribly biased. I was exceptional at picking up new languages as a teen, but let that knowledge decay into nothingness as an adult. I’d hate to have wasted such a useful talent.)
There is a pretty famous research paper called “Cross-language speech perception: Evidence for perceptual reorganization during the first year of life” by Werker and Tees that shows that infants are better at picking up subtleties of phonemes and they start focusing on the most heard language at around 9 to 12 months of age.
The paper is pretty old, published in 1984, but it was very influential at that time. Janet Werker has several other studies about language acquisition, so she might be a good name to start checking when you’re interested in that topic.
Language acquisition happens different in young children than in adults or older children. Linguists are not divided on this topic.
I think most immigrants I know have an accent when they speak my language, even if they have good grammar and vocabulary, and have been living here a long time.
My dad has been in the US far longer than his birth country of Japan, having moved in his 20s. His English is excellent in terms of vocabulary and grammar, but his accent is very strong. I, on the other hand, having been exposed to the language when young and living in Japan for just one year, have a much better Japanese accent than he does English but am far worse in all other aspects.