how children learn languages

As I read through chapters 1 and 2, I was taking notes in shorthand until I came to the following paragraph: "By the age of six months, an infant has produced all of the vowel sounds and most of the consonant sounds of any language in the world, including some that do not occur in the language(s) their parents speak. If children hear English spoken around them, they will learn to discriminate among those sounds that make a difference in the meaning of the English words (the phonemes) and they will learn to disregard those that do not". The section went on to explain that most children have mastered the distinct sounds of their L1 by age 3 and the grammar structures of their L1 by age 5-6, although more complex grammar structures may develop later as children get older. In reading this section, I learned a great deal about language development and the 'innate capacity', or natural ability, of all human beings to learn a language.

I had to copy down that entire paragraph in my notes because it was SO interesting to me. I think it is fascinating to study children's language development and how they come to know a language so well. Clearly, this process can never be perfectly replicated in neither formal nor informal settings later in life for those who attempt to learn an L2 at an older age. However, the paragraph also left me wondering about a certain situation I encountered a few months ago.

I know a family comprised of a Spanish-speaking mother and an English-speaking father. To my knowledge, the father has learned Spanish as his L2 and speaks it almost fluently, while the mother still speaks only Spanish and very limited English. Their son, however, presents a very interesting case. He is 2 or 3 years old and understands both English and Spanish. However, when spoken to in Spanish, he often chooses to respond in English. After reading this section of the book, I had to wonder how that affects his overall linguistic development. If he chooses to use only the English sounds, although he is consistently hearing the Spanish sounds, will that affect his ability to produce native-like Spanish later in life?

I think it is very interesting to study how to learn a second (or third, or fourth!) language among students of any age, but through even my very limited exposure to second-language learning, it is most certainly beneficial if one can learn an L2 at a young age, under exposure to a native speaker of the L2. This demonstrates how important the individual sounds are, even before the child can speak sentences or even words. I think this would be very difficult to reproduce in a formal learning setting for older students, if not impossible.