While I agree that adult capabilities for language learning are often underselled, you need to appreciate that a 2 year-old native speaker hasn't got the capabilities to learn the language to the full extent. Yet a 2-year old is likely to have a better phoneme representation – often much better – in their native language than an adult who has studied 2 years in language classes.

At a wedding I met a photographer from the far east who had learnt perfect English in his late teens with (to my native ears) a perfect northern accent. So it is possible.

I suspect that even a tin-eared adult learner could master those phonemes with enough obsessive repetition, both listening and speaking. I'm reminded of Liam Neeson's character in 'Taken' spending a transatlantic journey listening over and over again to the voice of his daughter's kidnapper just so he could recognise him later on.

That level of obsession requires motivation which I think is highly under-appreciated where learning is concerned.

We often think of small children as possessing magic learning brains without considering how highly motivated they must be. The ability to speak means they can more easily make their basic preferences and problems known to parents and so on. It's hard to imagine what it was like for us back in early childhood.

However, children do have one natural advantage: if you learnt a language when young then it must be harder to forget it, due to Ribot's Law:

https://www.youtube.com/watch?v=bYPTzw3iN5Q

Yes, it's possible. What I was trying to say, is that children get almost invariably past a certain level of goodness, whereas with adults, the results vary a lot. And I agree that motivation has a lot to do with it, as well as that children are very motivated.

I agree that for most adult learners, aiming for accent-free is likely to disappoint. However, my personal figure of merit is getting to the point where people are happy conversing with you in their L1/your L2, rather than wishing to swap to their L2/your L1, and any reasonably motivated adult ought to find that attainable.

  no effort: your L1, their L2
  easy mode: your L1, their L1
  aim for:   your L2, their L1
  hard mode: your L2, their L2

(indeed, in my experience —past an acceptable baseline— people judge L2 speech far more by register and rhetoric than by phoneme production)

> children are very motivated

That, and much less afraid that they "might make a mistake"

I suspect that environment has a lot more to do with that than innate capability.

A two-year-old lives in a full immersion environment where the only language they can communicate in is the one they are learning and where a large portion of the people they interact with are going out of their way to help them practice. They also have no responsibilities other than learning language and a few other skills.

Put an adult in an environment like that and they will make extremely rapid progress.