I think the age was around 9 months where some where the majority of neurons get discarded since they're not useful and babies can use them to identify different lemurs for example.

I still think this is not the whole picture. People in their 60s learn languages faster than teenagers because they have more "surface area for knowledge to stick to" if you know where I'm going.

Not to take anything away from babies learning languages. They have to learn to use their tongue, vocal chords and identify sounds at the same time so them being able to use grammar at 2y with vocabulary is as insane feet. Babies also have dedicated regions of the brain for speech but then again, so do adults.

I feel the post makes a valid point. It's harder for the baby to learn a language since they don't know any. An adult in the same situation would most likely be faster because majority of the skills needed to speak a language are already there.

About the accuracy of sounds it'll take some very careful training of the adult like people do with babies. On top of that, I wouldn't underestimate the brain, over a long time such as a year of full immersion the brain is capable of picking up on a lot of things as long as the adult pays attention.