Critical period hypothesis

What is it? The critical period hypothesis (CPH) states that language acquisition is linked to age. “The critical period hypothesis was first proposed by Montreal neurologist Wilder Penfield and co-author Lamar Roberts in their 1959 book Speech and Brain Mechanisms, and was popularized by Eric Lenneberg in 1967 with Biological Foundations of Language”. © Wikipedia

The hypothesis claims that there is an ideal period (2-13 y.o.) to acquire language at the native level. After this period further language learning requires strenuous mental exertion and effort and the student won’t be able to achieve full mastery of language. 

The theory has extended onto second-language acquisition (SLA). 

1. Critical Period Hypothesis has a “use it then lose it” approach. If an adult doesn’t practise L2, they forget it as the brain doesn’t need to spend energy to maintain unnecessary language structures. Whereas if L2 is learnt at an early age and is constantly practised, the language “remains active” in the brain. 
2. In addition, there are theories of Penfield and Roberts (1959) and Lenneberg (1967). They state that language acquisition happens mainly in childhood as the brain loses its ability to be “modified” after a certain age. It becomes inflexible and isn’t able to be changed or adapted. The scientists assert children under nine can learn many languages, early exposure to different languages allows them to switch between the language systems without confusion or translation. They also claim that if a language isn’t learnt by puberty, it cannot be learned in a normal, functional sense.
3. Another theory, Universal Grammar by Chomsky, implies that young learners acquire languages easier than adults, as adults have to activate principles developed during L1 learning and imitate Second Language Acquisition. The principles developed through L1 acquisition are vital for learning an L2. Whereas children can learn several languages simultaneously as long as the principles are active and they are exposed to sufficient language input. This theory asserts that older learners may have difficulty in learning the target language’s rules from positive input alone. 
4. Krashen, though he criticizes Critical Period Hypothesis, supports the importance of age for Second Language Acquisition. He claims that the ability to make abstract hypotheses hinders elder learners’ natural ability for language learning.
5. Another proof for Critical Period Hypothesis is bilinguals. Lots of research shows that Second Language Acquisition in early childhood provides children with a greater awareness of linguistic structures. Furthermore, they are not taught systematically but naturally. The efficiency of child’s Second Language Acquisition depends on different factors: their interest, learning environment, motivation, amount of exposure. Second Language Acquisition  broadens children’s outlook, they gain knowledge about the different cultures and environments.

However, this hypothesis is not universally accepted and some scientists suggest to call it a “sensitive” or “optimal” period, rather than a critical one.

1. Firstly, adults may have certain reasons why they do not acquire languages as good as children do. These reasons are not always about certain abilities, the evidence for L2 learning ability declining with age is controversial. The reasons can be: lack of time, finding their own appropriate learning style, motivation, setting realistic goals, choosing suitable methodologies, etc. 
2. Secondly, the evidence for a certain period is limited and is “proved” only by analogies of the same kind of periods in biology: physical maturation, cognitive factors. Moreover, there is no established and proven duration of the period.
3. Thirdly, the debates around the hypotheses are also due to the lack of the definition of language. Some aspects cannot be acquired after a certain period. For example, usually adults cannot get rid of L1 accent while younger learners speak with a close-to-native accent. This is due to the fact that pronunciation, or phonology, is influenced by the critical period. In contrast, there is no critical period for learning vocabulary. It can be enhanced by training at any age as it’s learned consciously using declarative memory which, however, differs from L1 learning when we use procedural memory. 

All in all, it can be concluded that:

1. there is no certain critical period for L2 learning.
2. both adults and young learners are able to achieve a near-native level in an L2 depending on their abilities, although kids may have some advantages.
3. certain characteristics of the learning environment have great importance as well.

You may also find these articles and resources useful to read:

1. Wikipedia, CPH
2. CPH by Anna and Kathrin Mall
3. CPH in ELT Journal
4. The Critical Period Hypothesis in Second Language Acquisition: A Statistical Critique and a Reanalysis
5. History of the CPH