This paper examines the effects of onset age of exposure to a dominant language (English) with respect to Korean heritage speakers’ knowledge of unaccusativity involving quantifier float. In an attempt to see how the acquisition of heritage grammar is affected by this factor, the present study compares two groups of heritage speakers: early (US-born, n = 13) and late (Korean-born, n = 14) bilinguals. The results show that compared to the late bilinguals, the early bilinguals did not give differential ratings to unergative and unaccusative verbs, which confirms the widely noted observation that the earlier onset age of exposure to English is, the more likely heritage speakers’ linguistic knowledge of the heritage language is incomplete. In addition, the results show that incomplete knowledge was also found with the late bilinguals (mean onset age 9), in that they did not accept unaccusative verbs as strongly as the native controls did.