Many of the research articles had limitations in the way ‘child’ and ‘adult’ are defined. Generally, according to the critical period hypothesis, participants under the age of 13 were considered as children, and participants above the age of 13 were considered as adults. However, the results of most of the research displayed stratified data for an in-between age group (13 to 18 years old). This affected the overall discussion of the results because this particular age group performed in ways that overlapped with children’s performance in L2 production and adult’s performance L2 production. This caused the data collected to be varied and stratified among the ‘adult’ age group (above 13 years old). Perhaps future research can propose a separate ‘adolescent’ group for the transitional age group to account for the stratified data.
For example, Karen Melissa Lichtman (2012) proposed an argument against the Critical Period Hypothesis – that the change from implicit language learning to explicit language learning does not only happen abruptly at the age of puberty. The change happens gradually and continuously throughout life. In this sense, the decline in the ability to achieve native-like proficiency in a language slowly declines with age, and not abruptly after the age of puberty, hence providing reason for a transitional ‘adolescent’ age group to be studied.