A broadly held, yet nearly as broadly criticised, view is that of the United States linguist Noam Chomsky in what he calls the human "'language acquisition device" '— a mechanism which enables an individual to recreate correctly the rules (grammar) and certain other characteristics of language used by speakers around the learner. This device, according to Chomsky, wears out over time, and is not normally available by puberty, which he uses to explain the poor results some adolescents and adults have when learning aspects of a second language (L2). 

If language learning is a cognitive process, rather than a language acquisition device, as the school led by Stephen Krashen suggests, there would only be relative, not categorical, differences between the two types of language learning.

Despite the differences in theories, most studies agree that the earlier children learn a second language, the better off they are, cognitively speaking at least. These studies could be used to make the learning of a second language mandatory in all schools as early as possible, in order to give children every means of increasing their cognitive abilities. Many European schools offer secondary language classes for their students, if for no other reason than the proximity of other countries with different languages. The United States, however, in spite of its proximity to francophone Quebec and hispanophone Mexico, is the only technologically advanced country that does not require the study of a foreign language in its schools.

Definition of multilingualism
Learning language
Multilingualism between different language speakers
Multilingualism at the linguistic level