Purpose: The impact of second language (L2) on first language (L1), known as L2 transfer, has been suggested as a fundamental driving force of L1 attrition. The goal of this study was to test the differential attrition of verb aspect and tense in L1 (Russian) under the influence of L2 (German) grammatical properties. We also investigated whether the age of bilingualism onset and the amount of exposure to L1 modulate this L2 transfer effect. Methodology: We tested sentence processing in 30 adult Russian monolingual participants and 30 L1 attritors – Russian-German bilingual speakers – with early versus late bilingualism onset and with low versus high amounts of exposure to L1. Participants heard grammatically correct sentences, sentences with aspect violations and sentences with tense violations, and were asked to detect errors. Data and Analysis: The accuracy of participants’ responses was analysed using generalized linear mixed-effects modelling in R. Findings: The L2 transfer effect was found, but was strongly modulated by the amount of L1 exposure: only bilinguals with little exposure to L1 showed greater attrition of L1 aspect compared to L1 tense. Moreover, the age of bilingualism onset proved to be more critical than the L2 transfer effect: an earlier bilingualism onset resulted in greater attrition of both aspect and tense in L1. Originality: The study provided new evidence about the differential impact of the grammatical similarity between L1 and L2, the age of bilingualism onset and the amount of L1 exposure on aspect and tense processing in L1 attritors. Implications: Our findings suggest that greater L1 use after immigration helps bilingual speakers to be less susceptible to L2 transfer and prevents attrition of L1-specific grammatical categories. Also, a general decline in processing verbal morphology is more likely to occur in speakers with an early rather than a late onset of bilingualism.