I'm pretty sure along with "children's brains being more flexible", this is pseudo-science. I've just had a quick look around the peer reviewed literature and can't find anything that support this, the only things I can find talk about this stuff in terms of 'myths'.
AFAIK, at birth a child is capable of making the sounds of any
human language, but they tend to learn, by trial and error like Pavlov's dog, only to keep making the sounds to which the people around them respond positively. The attention the child receives acts as a reward and they keep making those sounds which are rewarding for them.
Older humans' vocal organs lose the ability to make "every" sound. It is a rare adult who can speak other languages, learned as an adult, with perfect pronunciation and accent.
Pretty certain that's not a "myth".