The theory has often been extended to a critical period for second-language acquisition (SLA), although this is much less widely accepted. Certainly, older learners of a second language rarely achieve the native-like fluency that younger learners display, despite often progressing faster than children in the initial stages. David Singleton[10] states that in learning a second language, "younger = better in the long run," but points out that there are many exceptions, noting that five percent of adult bilinguals master a second language even though they begin learning it when they are well into adulthood—long after any critical period has presumably come to a close.

While the window for learning a second language never completely closes, certain linguistic aspects appear to be more affected by the age of the learner than others. For example, adult second-language learners nearly always retain an immediately identifiable foreign accent, including some who display perfect grammar (Oyama 1976). A possible explanation for why this foreign accent remains is that pronunciation, or phonology, is susceptible to the critical period.[11] The pronunciation of speech sounds relies on neuromuscular function. Adults learning a new language are unlikely to attain a convincing native accent since they are past the prime age of learning new neuromuscular functions, and therefore pronunciations. Writers have suggested a younger critical age for learning phonology than for morphemes and syntax. Singleton (1995)[12] reports that there is no critical period for learning vocabulary in a second language because vocabulary is learned consciously using declarative memory.[13] The attrition of procedural memory with age results in the increased use of declarative memory to learn new languages, which is an entirely different process from L1 learning. The plasticity of procedural memory is argued to decline after the age of 5. The attrition of procedural memory plasticity inhibits the ability of an L2 user to speak their second language automatically. It can still take conscious effort even if they are exposed to the second language as early as age 3. This effort is observed by measuring brain activity. L2-users that are exposed to their second language at an early age and are everyday users show lower levels of brain activity when using their L1 than when using their L2. This suggests that additional resources are recruited when speaking their L2 and it is therefore a more strenuous process.

The critical period hypothesis in SLA follows a "use it then lose it" approach, which dictates that as a person ages, excess neural circuitry used during L1 learning is essentially broken down.[14] If these neural structures remained intact they would cost unnecessary metabolic energy to maintain. The structures necessary for L1 use are kept. On the other hand, a second "use it or lose it" approach dictates that if an L2 user begins to learn at an early age and continues on through his life, then his language-learning circuitry should remain active. This approach is also called the "exercise hypothesis".[15]

There is much debate over the timing of the critical period with respect to SLA, with estimates ranging between 2 and 13 years of age.[13][16] These estimates tend to vary depending on what component of the language learning process a researcher considers. For instance, if an SLA researcher is studying L2 phonological development, they will likely conclude that the critical period ends at around age 3. If another SLA researcher is studying L2 syntactical development, they may conclude that the critical period ends at a much later age. These differences in research focus are what create the critical period timing debate.

Some writers have argued that the critical period hypothesis does not apply to SLA, and that second-language proficiency is determined by the time and effort put into the learning process, and not the learner's age.[16][17] Robertson (2002)[18] observed that factors other than age may be even more significant in successful second-language learning, such as personal motivation, anxiety, input and output skills, and the learning environment. A combination of these factors often leads to individual variation in second-language acquisition experiences.

On reviewing the published material, Bialystok and Hakuta (1994) conclude that second-language learning is not necessarily subject to biological critical periods, but "on average, there is a continuous decline in ability [to learn] with age."