Child L2 learners are less likely to produce ungrammatical sentences as compared to adult L2 learners, especially in morphological errors such as the incorrect addition or omission of the plural -s in English (Finn et al., 2014). Adult L2 learners also tend to produce entire phrases or sentences that are inconsistent with the internal phrase or clause structure, as compared to child L2 learners (Finn et al., 2014).
In a study conducted by Becky H. Huang (2013), it was found that the age at which L2 learners acquire the L2 affects grammaticality judgement. In the study, the participants involved comprises 118 Mandarin-English bilingual immigrants in the United States (English is the L2), and 24 English native speakers from the United States. The native speakers were included as they were the control group. The bilinguals spoke only Mandarin before they were 5 years old. This was to ensure that English was indeed the L2. Also, before the bilinguals arrived in the US, they did not have much exposure to English (e.g. being enrolled in English-medium international schools). The bilinguals had also lived in the US for a minimum number of 6 years, with at least 3 consecutive years. This is to ensure that they have attained some level of acquisition of the L2. The last criteria that the bilinguals have met was that they were not diagnosed with hearing issues, language disorders, or learning disabilities.
All the participants took a grammaticality judgement task. In the task, 84 English sentences were taken from past studies on English morphosyntax acquisition. Of which, there were 42 pairs of grammatically correct and incorrect pairs, and the sentences contained parts of speech found in standard English grammar. Sentences were presented to participants one at a time on a laptop. Participants were asked to judge whether or not a sentence was grammatical via keyboard response on the laptop. The results of this study showed that L2 learners in general performed poorer than the native speakers, as predicted. Within the L2 learners, bilinguals who arrived in the US at a younger age (and thus was exposed to English at a younger age) performed better at the grammaticality judgement test, as opposed to the bilinguals who arrived in the US after the age of 13. The former group managed to correctly judge grammatical and ungrammatical sentences on an average of 84% of the time. Some of them have also achieved near-native proficiency in grammar, with a score of about 90% accuracy (native speakers had an average score of 95% accuracy). Participants who arrived in the US after the age of puberty managed to correctly judge the grammaticality of the sentences on an average of 62% of the time. These results then show that children are more able to attain native-like proficiency in L2 grammar, as compared to adults.