The effect of language aptitude on grammar attainment of second language learners with different ages of onset became one of the hot topics in the last decades. However, most previous studies were conducted in a second language context with inconsistent results, and few studies were conducted in a foreign language context. To fill this gap, the current study investigated the impact of language aptitude on grammar attainment of EFL learners with different ages of onset in the Chinese context. A Timed Grammatical Judgment Test and an Untimed Grammatical Judgment Test, a Metalinguistic Knowledge test, and a Language Aptitude test were used in the study, and 112 participants were sampled. The results show that the aptitude scores of the three groups of EFL learners with different ages of onset have no significant difference; language aptitude moderates performance on the Untimed Grammatical Judgment Test and the Metalinguistic Knowledge test, which was mainly reflected in the earlier starters; and higher aptitude learners outperform lower aptitude learners in all three grammar tests. Overall, this study provides evidence to support the impact of language aptitude on EFL learners in the Chinese context.