Speaking from my experience: I've just recently started to work with a phoneticist to improve my English accent and those lessons opened a whole new world of ho

Speaking from my experience: I've just recently started to work with a phoneticist to improve my English accent and those lessons opened a whole new world of how people speak to me. I've started to discern a ton of new sounds, hearing things I'd never been able to hear before (e.g. how æn is almost a diphthong in American English). I hadn't any special training earlier in my life (I'm 26 now), so I think an ability to discern phonemes in a foreign language can be acquired while being an adult at least in some cases.