This study examines the role of age of arrival (AoA) in immigrant students' perceived multilingual proficiency. Additionally, the study investigates the demographic, linguistic and social-psychological variables distinguishing young-arriving immigrants from their middle and older-arriving peers. The sample included 274 eleventh grade Russian (L1) speaking immigrants acquiring Hebrew as a second language (L2) and English as a third language (L3), divided into three groups according to arrival ages in Israel: 0-6; 7-9 and 10-12 years. The instruments included a 'can-do' questionnaire measuring multilingual self-assessment, (SA) and a background questionnaire. The results indicated a strong relationship between AoA and students' SA in L2 and L3 extending research, based primarily on language tests, demonstrating that AoA is an important factor explaining multilingual proficiency. Further, the findings revealed that students' self-assessed listening in L2 and L3 distinguished the young-arriving immigrants from their middle and older-arriving peers. Current use of L3, parental encouragement to study L2 and an inverse integrative orientation to study L3 distinguished the older-arriving group from the other two groups. These results highlight the role of the sociolinguistic context and the specificity of each language as a source of variation in L2 and L3 acquisition besides individual characteristics.