The issue of critical or sensitive periods affecting the outcome of second language (L2) acquisition has been the subject of intense investigation and debate for many years, with people arguing for or against maturational effects on ultimate attainment. In their influential paper, Johnson and Newport (Reference Johnson and Newport1989) identify two hypotheses: the exercise hypothesis and the maturational state hypothesis. According to the former, if the capacity for acquiring language is exercised early in life (in first language acquisition), then language learning abilities will remain intact throughout life: in other words, permitting successful L2 acquisition regardless of age. In contrast, according to the maturational state hypothesis, the language learning capacity declines with age, affecting L2 acquisition as well as late L1. Johnson and Newport take their results, which show an age-related decline in performance during childhood and adolescence, to support the maturational state hypothesis. Many L2 researchers have adopted a maturational perspective and have reached similar conclusions as to the presence of critical or sensitive periods (e.g., Abrahamsson, Reference Abrahamsson2012; DeKeyser & Larson-Hall, Reference DeKeyser, Larson-Hall, Kroll and de Groot2005; Long, Reference Long1990; Oyama, Reference Oyama1976; Patkowski, Reference Patkowski1980). Some researchers have pointed out that age effects continue into adulthood, contrary to the claim for a critical period (e.g., Birdsong & Mollis, Reference Birdsong and Molis2001). Others have suggested that what looks like an age-related maturational decline may be accounted for by confounding factors, such as task effects, effects of L1, amount of L2 use, education or input (e.g., Bialystok & Miller, Reference Bialystok and Miller1999; Flege, Yeni-Komshian & Liu, Reference Flege, Yeni-Komshian and Liu1999).

Mayberry and Kluender (Reference Mayberry and Kluender2017), as well as Mayberry and colleagues in earlier work (e.g., Mayberry, Reference Mayberry1993; Mayberry & Lock, Reference Mayberry and Lock2003), have been amongst the few to explore the other possibility, namely the exercise hypothesis. In a number of studies, Mayberry and colleagues have compared L1 and L2 learners of ASL, showing that, when age of acquisition is controlled for, L2ers outperform late L1 learners in a number of respects. In addition, native signers acquiring (written) L2 English perform like hearing learners in demonstrating considerable success in L2 acquisition, in contrast to late L1 acquirers learning an L2. In other words, as these researchers have demonstrated, a crucial predictor of L2 success is timing of L1 acquisition.

A central issue that faces both the exercise and the maturational approaches is how to define successful outcomes, how to determine the nature of ultimate attainment. In the literature on age effects in L2, it is usually taken for granted that success is to be defined in terms of convergence on the L2 grammar (or ‘target’ language); hence, monolingual native speakers provide the comparison group. Typically, researchers have picked a number of linguistic phenomena and have compared the performance of L2ers to native speakers, showing that L2ers either do or do not converge on native speaker performance on the chosen properties. Failure to converge is taken to indicate some kind of deficiency in the L2ers’ ultimate attainment.

However, it is important to bear in mind that the acquisition task involves coming up with a linguistic system that allows the learner to use the L2 (in comprehension and production). The task is not to arrive at a grammar identical to that of a native speaker. As a number of researchers have pointed out, comparing L2ers to monolingual native speakers is not necessarily the most appropriate way to establish their unconscious knowledge of the L2 or the nature of their ultimate attainment. Rather, the linguistic systems achieved by L2ers need to be considered in their own right (e.g., Bley-Vroman, Reference Bley-Vroman1983; Cook, Reference Cook1997). While interlanguage grammars may differ from those of native speakers, this does not make them defective or any less systematic than a native speaker grammar. One interesting possibility, then, which the exercise hypothesis allows one to entertain, is that late acquired L1s are indeed in some sense defective (or incomplete, a term sometimes used in the context of heritage language acquisition (e.g., Montrul, Reference Montrul, Lefebvre, White and Jourdan2006)) whereas late acquired L2s are not. Constructing a complex linguistic system may not be possible in late L1 acquisition but it certainly is possible in late L2.