[FranzFerdiNaN]

> but as an adult, even a decade of effort won't get him to the level any Danish person can speak English.

Children don't have any special ability to pick up languages, but they do have certain advantages: they aren't afraid to make mistakes, they are fully immersed in the language from the moment they wake up, they have no responsibilities like jobs and other adult stuff and they have people (parents, school, teachers, family) constantly talking to them at a level they can comprehend while gently pointing out their mistakes. If you put that barber in the same environment he will be fluent in no time.

[ido]

I believe they do have an advantage, mental plasticity is higher in children (especially young children).

[SkyBelow]

>Children don't have any special ability to pick up languages

They do. One example I recall from a neurology class is that the auditory cortex of a young child is able to differentiate more sounds than that of an adult. As a child ages, they lose the ability to distinguish sounds not common in their native language. The inability to distinguish the sounds also applies to the inability to tell the sounds apart when speaking it which decreases their ability to produce the correct sound. There is some research showing it is possible to relearn the ability to differentiate, but this requires training specifically in identifying sounds, not training in learning a language.

This doesn't mean fluency as an adult is impossible, and there are many actions that an adult could take to better align their learning environment to that of a child's which would improve their learning, but we should recognize there are some difference that cannot be replicated in adults.