The most important thing to consider is what are the exact factors that facilitate L2 learning as a rule. They are:
- cognitive ability
- sociocultural malleability and interaction
- affective filter (emotional maturity)
- linguistic acumen
This being said, research currently focuses on identifying a prime developmental stage that may unite all of these facilitators. Even after years of research, the answer remains the same: it is not an "age" but a "circumstantial" issue. If a student unites all of the above-mentioned enhancers, language learning can occur at just about any age.
Now, the general belief is that children acquire language better than adults, or that children learn faster because of their fast-developing brains. This belief comes as a result of Lenneberg's Critical Period Hypothesis, which cites neuroplasticity and lateralization as the two processes that occur most between the ages of 2 until puberty. Hence, Lenneberg claims that, after puberty, L2 acquisition is much harder to acquire.
Yet, Krashen, Gardner, Long, and Freeman, among others, have counter-argued Lenneberg citing, among other things, that different studies have shown a huge range of diversity in language ability from different age levels. Also, lateralization is a theoretical process while neuroplasticity may or may not be a factor involved in the process of retaining specific instances of language such as figurative expressions, irony, and overall prosody. The concluding fact, to this day, continues to be that linguistic ability is quite dependent on both natural and environmental factors: there has to be ability, continuous exposure, the attitude and motivation to learn and, most importantly, an ample amount of opportunities to apply it.
If language were a product of automatic brain functions, people would never forget their native language, yet, many people actually do. Therefore, as with any type of learning, L2 has to be an ongoing and rich process for it to transfer, and remain, for purposes of long term memory and usability.
Language acquisition is something we all take for granted. Any deficiency in early acquisition of a first language is usually considered an area for a speech therapist to get involved to ensure that the "problem' is rectified.
We expect speech development to be a natural process and, generally, support the theory that
humans are neurologically "programmed" from birth with the capacity to acquire language as soon as their nervous system reaches a certain point of maturation.
There are many factors which affect the development of language, whether first or second or additional languages. In homes where parents are fluent in more than one language, the second language becomes more of an additional language. Young children develop very successful language skills in both languages as long as parents do not confuse the two languages, having a preferred first. Individual parents may speak in the second language only on weekends, or one parent may favor one language and the other parent the other language, and so on.
In instances where the second language is a foreign language or a language acquired for educational purposes, cultural or travel purposes or perhaps religious purposes, the task becomes much harder. Then, from a spoken-language perspective, it is children in the older groups who feature better in the short-term but, interestingly, the younger learners do eventually surpass the older learners.
An interesting point (as noted by Cummins) is a recognition of stark differences in competence between conversationally spoken proficiency and isolated language learning:
face-to-face conversational proficiency and proficiency which requires the speaker to rely solely on the language itself (Cummins:1979; 1980; 1981a; 1981b)
In contrast to the oral skills development of second language learners and younger children, from the perspective of academic language – including complex grammar and so on, older children are and remain ahead of younger children (aged from about 4 – 8). One of the explanations for this distinction is that the younger group have not yet entrenched their first language academic skills.
When it comes to adults and second language acquisition, there is a major emphasis on pronunciation and accent which is unfortunate as this does hamper the learning process. Anyone listening to an adult speaking a newly-acquired language, cannot help but be influenced by the inflection in the voice and the inevitable 'mistakes' which others feel duty-bound to correct. Even if the language is correct, the pronunciation gives an adult a feeling of inferiority and reduces their likelihood of using the language.
As practice makes perfect, it is then difficult for an adult to perfect their second language.