The present study examined whether early immersive L2 exposure in a foreign language learning context can yield long-term advantages in L2 morpho-syntactic sensitivity. Participants were 40 Korean university students with high English proficiency, who had either attended an English kindergarten or begun learning English in a classroom, and a control group of native English speakers. All participants performed a speeded aural grammaticality judgment task that included the following features: articles, subcategorization, plural -"s," third-person -"s." Results showed that the English-kindergarten group outperformed the late-classroom group in terms of accuracy for ungrammatical sentences, while the two groups did not differ significantly on grammatical sentences and in reaction time measures. The learners altogether scored higher in plural -"s" and third-person -"s" compared to articles. While the native speakers showed near-perfect accuracy and fast reaction times, the highly proficient learners were at near-chance level in detecting morpho-syntactic errors during online L2 aural processing.