The current project examined whether and to what degree age of acquisition (AOA), defined as the first intensive exposure to a second language (L2) environment, can be predictive of the end state of postpubertal L2 oral proficiency attainment. Data were collected from 88 experienced Japanese learners of English and two groups of 20 baseline speakers (inexperienced Japanese speakers and native English speakers). The global quality of their spontaneous speech production was first judged by 10 native English-speaking raters based on accentedness (linguistic nativelikeness) and comprehensibility (ease of understanding) and was then submitted to segmental, prosodic, temporal, lexical, and grammatical analyses. According to the results, AOA was negatively correlated with the accentedness and comprehensibility components of L2 speech production, owing to relatively strong age effects on segmental and prosodic attainment. Yet significant age effects were not observed in the case of fluency and lexicogrammar attainment. The results suggest that AOA plays a key role in determining the extent to which learners can attain advanced-level L2 oral abilities via improving the phonological domain of language (e.g., correct consonant and vowel pronunciation and adequate and varied prosody) and that the temporal and lexicogrammatical domains of language (e.g., optimal speech rate and proper vocabulary and grammar usage) may be enhanced with increased L2 experience, regardless of age.