Acquisition vs. learning

What differences are there between the way that a child learns his/her first language, and the way an adult learns a second language?

“[…] The proper goal for an L2 user is using the language as an L2 user, not like an L1 user, with the exception […] of those who want to be spies.” ,Cook.

Many writers have expressed their views on the differences between language acquisition and languagelearning. We say we acquire a language through our childhood, and we are able to master it in later stages. Learning a language is a far more complicated process: our brains already have the knowledge of a previous grammatical system.
When acquiring a language, many factors have to be taken into account. As Moon et al. explain in their article Two-Day-Olds Prefer Their Native Language, it hasbeen proved that children “choose” their mother's language as favourite when compared to another one, in this case, Spanish and English. They did a research on how two-day-old babies perceive language and it showed that they responded with sucking at a faster rate to the sounds they associated to their mother's language than to those sounds which were not familiar to them.
This proves that,somehow, children do not choose the language they will use in the future. The speaking environment in which the pregnant mother is would be crucial to the development of the baby's Universal grammar. They are ready to activate the same linguistic patterns and sounds that appear in their environment, just because they are familiar with the sounds these languages have.
Pinker holds in many of hiswritings this same idea: children have an innate ability to memorize vocabulary and internalize grammar, since they are surrounded by speakers whose awareness of the L1 grammar is complete. However, it is not until they reach the age of 6 to 8 when they have a complete, full knowledge of the grammar, although some errors, easily perceived by adults, may still appear. Before this last stage, childrenmay have gone through other phases, being the first phase the most important one: baby babble, which is essential, as it familiarises babies with the sounds of their voices and they start to gain control over their own vocalizations. They will learn simple words, such as mama or water, to make requests, to demand or exclaim, before attempting with real communication. During this second stage,children will join words in short, grammatical strings such as give milk, or daddy home.
However, he also points out that there is a certain age up to which children stop learning. This is due to the “linguistic plasticity” of the brain. He affirms that there is a period during which children can learn the language used in his/her environment, since it has a strong influence in the acquisition of anL1, also called “positive evidence”: a Japanese child is not more able to speak Japanese than a French child, but he/she will speak the language that is found around him/her. The period when we can find this linguistic plasticity lasts up to the age of eight, approximately. If a child does not learn a language after this age, he/she will probably never learn to speak properly any language. We cansee this happening with the so-called “wolf-children”. Those found before the age of eight have succeeded in learning the language. On the other hand, children like Genie, who was found in 1970 in Los Angeles at the age of twelve, do not develop a complete grammar. This girl could only recognise twenty short words, most of them with a negative meaning, although she could distinguish differentwords when relating them to pictures.
The learning of a foreign language can be a demanding experience, especially learning to write and speak. The “window of opportunity” is larger and more variable, although people who learn a language after puberty are less likely to master these two skills. Nonetheless, if someone learns multiple languages before puberty, he/she will be able to speak both (or...