This paper addresses the instability of the lexicon-syntax interface in bilingual L1 and L2 acquisition, as well as the effects of the age of first exposure to the L2 on the ultimate attainment of this interface. The phenomenon under scrutiny is auxiliary selection with intransitive verbs in Italian, whose orderly gradience is captured by the Auxiliary Selection Hierarchy. Child and adult Croatian advanced/near-native L2 speakers, Croatian-Italian simultaneous bilinguals and Italian L1 speakers performed an acceptability-judgment task. Bilingual L1 and L2 speakers proved to be sensitive to the gradience in auxiliary selection with intransitive verbs, but to have less determinate intuitions than monolingual L1 speakers, which is interpreted as evidence for the instability of the lexicon-syntax interface in non-monolingual development. In addition, adult L2 speakers proved to be less sensitive to the gradience than child L2 speakers, pointing to an interaction between the age of first exposure to the L2 and the acquisition of the lexicon-syntax interface.