No one said the brain works tha same at all ages. I believe it probably works better when you are an adult.
This is the reference you should have posted when you said that babies were better than adults:
http://en.wikipedia.org/wiki/Theory_of_c...evelopment
"According to Piaget, the Pre-Operational stage of development follows the Sensorimotor stage and occurs between 2?7 years of age. In this stage, children develop their language skills."
That's true. Some do earlier, some do later, but Piaget ignores second language acquisition, nor says that one's ability to learn a language is compromized after this stage. Actualy he says the oposite:
http://en.wikipedia.org/wiki/Critical_Period_Hypothesis
"Piaget (1926) is one psychologist reluctant to ascribe specific innate linguistic abilities to children: he considers the brain a homogeneous computational system, with language acquisition being one part of general learning."
This is the second one you should have posted:
http://en.wikipedia.org/wiki/Vygotsky#Th...d_Language
"Perhaps Vygotsky's most important contribution concerns the inter-relationship of language development and thought. This concept, explored in Vygotsky's book Thought and Language, (alternative translation: Thinking and Speaking ) establishes the explicit and profound connection between speech (both silent inner speech and oral language), and the development of mental concepts and cognitive awareness."
There is another passage on wikipedia that I find specialy interesting:
"Although Vygotsky believed inner speech to develop from external speech via a gradual process of internalization, with younger children only really able to "think out loud," he claimed that in its mature form it would be unintelligible to anyone except the thinker and would not resemble spoken language as we know it (in particular, being greatly compressed)."
So english is an extra skill, separated from thought, tought deeply linked.
But even with that, somehow, some people came up with something called the "critical period hypothesis"
http://en.wikipedia.org/wiki/Critical_Period_Hypothesis
"The Critical Period Hypothesis refers to a long-standing debate in linguistics and language acquisition over the extent to which the ability to acquire language is biologically linked to age. The hypothesis claims that there is an ideal 'window' of time to acquire language in a linguistically rich environment, after which this is no longer possible."
Notite the word _debate_ because there is no one ever proved it. It is stil an _hypothesis_.
For instance, I find the next passage pretty interesting:
"David Singleton (1995) states that in learning a second language, "younger = better in the long run," but points out that there are many exceptions, noting that five percent of adult bilinguals master a second language even though they begin learning it when they are well into adulthood ? long after any critical period has presumably come to a close."
Finaly the stone in the shoe of the critical period hyphotesis!
I'll finish this post here and let you read and reach your own conclusions.