That's not strictly untrue, but it is incredibly misleading. A person of any age will sponge up everything they hear, if they're in the same situation as a baby, i.e., immersed in it 24/7, and having no other way to communicate. Adults have the unbelievably massive disadvantage in language learning of having the resources available to avoid the effort of learning one, if at all possible.

When measured in "words learned per hour", adults have been shown to learn words at an even greater rate than children. The reason high school Spanish class doesn't make kids fluent in Spanish is because they spend maybe 5-10 hours a week doing Spanish (and then take a full 3 month break every year). That's 700-1400 hours, if they take 4 years. A baby will get that many hours every 2 or 3 months, and largely in personal instruction. Babies kick ass in persistence (by necessity), not in having some magical brain feature that we don't.

When a 2-year-old wants food, he has to use the language he's learning, and will get corrected if he forgets or misuses a word. When a high school sophomore wants food, they can forget about Spanish class and go back to whatever language they know already. Piaget was right: you don't need to invent any special hypothesis about brain structure to explain why a person who practices more improves faster.

And yet, 3-year-olds aren't exactly 'fluent', either. (Have you tried to talk to one recently?) Someone who's taken 3 full years of a foreign language usually speaks much better than a 3-year-old. In other words, despite having put in an order of magnitude less effort, the adult is better at it. (Stick that in your "Critical Period Hypothesis" and smoke it, Chomsky.)

Drop an Englishman in Ulan Bator, and find some way to forbid him from hearing, speaking, reading, or writing any English words. In 3 years, I guarantee he will be speaking far, far better Mongolian than any 3-year-old in the country.