Your suspicion is essentially that the critical period hypothesis is false. Linguists have been examining this confounding hypothesis for decades. As usual, mos

Your suspicion is essentially that the critical period hypothesis is false. Linguists have been examining this confounding hypothesis for decades. As usual, most of the research is paywalled, but here's [1] a decent summary of some of the research and evidence as of 1999. [1] https://web.stanford.edu/~hakuta/www/research/publications/(1999)%20-%20CONFOUNDED%20AGE%20LINGUISTIC%20AND%20COGNITIVE%20FACTORS%20IN%20.pdf https://web.stanford.edu/~hakuta/www/research/publications/(...