Primary language development begins at age 5 and is complete by puberty.
Here are the theories of primary language acquisition:
Behaviorist Theory: Language is acquired through enforcement and imitation, i.e.it is a form of operant conditioning.
Nativist Theory: Humans have an innate language acquisition device. Once vocabulary is learned, the learner has an innate ability to form meaningful sentences.
Empiricist Theory: There is enough information in what a child learns to allow development of the ability to communicate in proper sentences. There is no need for an innate language acquisition device.
Interactionist Perspective: This is a combination of nativist and behaviorist theories.
Second language acquisition follows the same steps in all adult students, but there is extreme individual variation in time for each step. Also, upon acquisition of fluency, there will be a considerable variation in level of fluency, writing ability, degree of non-native accent when speaking, and ability to enter advanced fluency such as expressing and understanding sarcasm and humor.
Stages of second language acquisition in adults are as follows:
Pre-production: Student acquires a vocabulary of up to 500 words, but does not speak, yet can repeat what teacher says.
Early production: This stage lasts approximately 6 months. The student has an active and repeatable vocabulary of 1000 words. She speaks in phrases of a few words.
Speech emergence: Vocabulary is 3,000 words. Student communicates in simple phrases and sentences.
Intermediate fluency: 6,000 words. Student uses more complex sentences in writing and speech. At this stage the student uses strategies from her native language to augment abilities in the second language.
Advanced fluency: This takes 4-10 years. Student has a near native skill in learning content, but needs assistance in studying history or social studies, and some assistance in writing.