I once spent nearly an hour trying to show five Korean and Japanese adults the difference between English… British… European, come to that… “L” and “R” and not one of them could hear it, though we breathed on balloons and touched each other’s throats. Developmental (whatsit)ologists suggests sounds not used before about age two - as different “L” and “R” - might be impossible for adults to learn, while “mere” emphases are merely tricky.
The cited comment might have been anecdotal, but I'm wondering what researchers have found about this.
What do specialists say about adults' ability to learn sounds of completely unrelated languages they have never heard before? Ability to recognize them after hearing them? Can any references be provided for that? By completely unrelated languages I mean, for example, Korean or Japanese and English, but not, for example, English and Persian, since they both derive from Proto-Indo-European.