I think you're mistaking "native fluency" for just fluency, which is probably my fault. Of course adults can become fluent in new languages relatively easily wi

I think you're mistaking "native fluency" for just fluency, which is probably my fault. Of course adults can become fluent in new languages relatively easily with full immersion, but the simplest things for native speakers can be the hardest to acquire. For example, before a baby can talk, they will babble in the intonation of their native language-- it's the first thing they learn. It's also the last thing a non-native speaker picks up, happening well after they've acquired full fluency, or indeed never in many cases. Now it may be that's more about our techniques for second-language acquisition than brain structure differences, but it's sure interesting, innit?