Learning a new language is hard if you're starting as an adult. But children do it easily, almost instinctively. You're not even trying to consider my point here. You're just restating your preconceptions rather than questioning them.
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'.