Does age affect the language acquisition process?

Yes, age seems to affect language acquisition, but researchers disagree about how much and why. Learners who start early and stay immersed in a language for many years usually outperform those who start later. The Critical Period Hypothesis explains this as a biological window for language learning, yet scholars still debate its exact scope and predictions.

Do younger learners acquire languages better?

First language speakers of any language are exposed to their mother tongue in the earliest stages of life. In immersion contexts, some second language learners also start the acquisition process early in life and remain exposed to input for years or even decades. In the long term, these early starters tend to outperform later learners, and this general age effect is widely accepted.

However, this age effect is sometimes misinterpreted as an argument for starting second or foreign language lessons early at school, even though classroom instruction takes place in very different circumstances from long-term immersion. The age effect is also sometimes taken as evidence for a so-called critical period (CP) for second language acquisition (SLA). The difference between natural acquisition and classroom learning is explained in our essay on mother tongue vs second language.

What is the critical period hypothesis?

The idea of a critical period comes from biology. It was famously introduced into the field of language acquisition by Penfield and Roberts in 1959 and refined by Lenneberg eight years later, in 1967. Lenneberg argued that language acquisition needed to take place between the age of two and puberty, a period which he believed coincided with the lateralisation of the brain, the process by which certain functions become based mainly in one side of the brain. Some more recent neurological research suggests that different language functions may have different time frames for lateralisation, although most of these are thought to close before puberty.

To support his claims, Lenneberg drew mostly on findings about first language development in deaf children, so-called feral children and children with serious cognitive impairments. For him, the critical period concerned implicit, “automatic” acquisition in immersion contexts. It did not rule out the possibility of learning a foreign language after puberty.

The exercise hypothesis and the maturational state hypothesis

Johnson and Newport (1989) offered two different interpretations of the critical period hypothesis (CPH), and each one has different implications for the role of age in second language (L2) acquisition.

According to the exercise hypothesis, children will be superior to adults in acquiring a first language. If people are not exposed to a language during childhood, they will not be able to acquire one fully later in life. However, as long as they have acquired a first language in childhood, their ability to acquire an L2 remains intact and can be used at any age. On this hypothesis, L2 acquisition should be equivalent in children and adults, so there should be no age effect.

In contrast, the maturational state hypothesis claims that there is something special about the maturational state of a child’s brain, which makes children particularly good at acquiring any language, both a first language and an L2. Over time, this special state does not stay intact, and the ability to acquire an L2 declines with maturation, whether or not it has been exercised. On this hypothesis, age effects should be visible in L2 acquisition.

How does the critical period hypothesis apply to second languages?

SLA researchers adopted the critical period hypothesis and applied it to second and foreign language learning, which resulted in a large number of studies. In its general version, the CPH for second language acquisition states that sensitivity to language input changes with age, and that adult L2 learners are less sensitive to input than child L2 learners. Importantly, this change is thought to be non-linear: sensitivity does not simply fall at a steady rate as learners get older.

Why is the critical period hypothesis still debated?

Beyond this general version, the CPH has been defined in many different ways. This means that researchers must state their theoretical position and assumptions clearly. It also has an obvious downside: critical findings can be played down as a problem for only one version of the CPH, while the other versions remain untouched.

As several researchers have pointed out, this vagueness affects two areas in particular: defining the scope of the CPH and stating predictions that can be tested. Both are basic steps in the scientific evaluation of any hypothesis or theory, but the lack of agreement among scholars on these points seems especially strong in the case of the CPH.

What does this mean for adult learners?

The debate does not mean that adults cannot learn a new language. Even Lenneberg did not rule out learning a foreign language after puberty, and the exercise hypothesis predicts no age effect at all for learners who already have a first language. Adult learners can make steady progress with regular input and practice, and they can track that progress against the six CEFR level descriptions, from A1 to C2. Confidence also plays a role, as our essay on speaking in an ESL context explains. To find your starting point, try our English level test.