[eitally]

I only have this anecdote. My wife, two kids (3 & 5 years old) and I have been taking Mandarin classes for the past six months. My wife & I pick up the concepts, grammar/patterns and vocabulary about three times faster than our kids (we are also forced to work harder at it because we're constantly reinforcing the lessons to them, so that inherently biases this analysis). That said, their pronunciation is muuuuuuuch better than ours.

[tomerico]

Kids learn by observing and interacting in the language. Adults usually learn the grammar and vocabulary directly, which is a slower mechanism for the brain.