This hypothesis was put forward by Eric Lenneberg in 1967. It states that children have a special propensity for acquiring language, especially between the ages of 2 to 13. There are two versions of CPH. While the strong one suggests that children must acquire their first language by puberty or they will never be able to learn from subsequent exposure, the weak one holds that language learning will be more difficult and incomplete after puberty.

Many researches show that age is a positive advantage. Researchers have studied situations where the opportunities for learning are similar for young children and adults. In many cases, they have found that older learners seem to learn more efficiently.

So far we have seen that there has not been sufficient evidence to support the Critical Period Hypothesis in language acquisition. Some adult foreign language learners have succeeded in acquiring a high level of a foreign language proficiency even at the age of 40. There are also examples in which age is a positive foreign language-learning factor. Many Chinese adult foreign language learners have difficulty or fail in the foreign language learning probably because of non-age factors, and even non-psycholinguistic factors. Linguistic differences between Chinese and English, cultural differences between the two languages' speakers may make part of the difficulties. Young children may have advantage over older children and adults in acquiring pronunciation skills. But, if the aim of learning a foreign language is not to pronounce like native speakers, but to communicate with them, this can be neglected.

So according to the critical period hypothesis and some related researches, different ages have different advantages in foreign language learning. During the puberty period, we should put emphasis on pronunciation or spoken language in foreign language learning. During adulthood, we can learn more about grammatical factors.