Many age-related second language (L2) studies have confirmed that young children have a better chance to become nativelike in L2 acquisition than adults. The current study investigated whether age effects exist in the L2 acquisition of Japanese and whether nativelike proficiency is guaranteed for early child L2 starters after constant target language (TL) exposure. The TL proficiency of L2 learners (n = 47) first exposed to the TL at various ages was examined using (i) impressionistic judgments of overall proficiency, (ii) relative clause (RC) comprehension, and (iii) RC production tasks in experimental settings. The RC tasks were designed to linguistically challenge the L2 learners, many of whom appeared to be nativelike in the impressionistic judgments. The impressionistic judgments confirmed the advantage afforded to earlier L2 starters, while the RC tasks revealed that nativelilke proficiency is not guaranteed even in early child L2 acquisition. Some child L2 starters in the current study qualitatively differed from native speakers in the RC production task even after many years of rich and sustained TL exposure.