simplyeric:

There is substantial literature that suggests that language acquisition is more difficult, slower, and ultimately less successful in adults than in younger people. I believe it's often mentioned that it's at about 12-14 yrs old that the neural plasticity for language acquisition fades.

I think the confusion here is about what exactly is meant by "learning" a language.

If the goal is to quickly build some rudimentary skill for finding one's way around in a foreign language (which basically boils down to memorizing a lot of words and stock phrases, along with a few very basic syntactic patterns), an intelligent adult will likely be able to achieve it faster because of better work ethic and superior general experience in tackling intellectual problems.

On the other hand, if the goal is to become indistinguishable from a native speaker, then the kid clearly has an advantage no matter how long it takes, because the task is impossible for the overwhelming majority of adults (which for this purpose means anyone over 12 or so). You may become a fluent speaker and perhaps even a good writer, but you're stuck with a foreign accent, and even if you manage to get rid of it with special training, you'll still make occasional subtle but noticeable syntactic and semantic errors.

If the goal is something in-between, the winner will depend on the exact benchmarks of success.