The impact of an individual's first language (L1) on their acquisition of a second language (L2) is widely recognized in the field of psycholinguistics. However, previous research has focused on limited L1-L2 pairs, leaving questions about how different linguistic structures, such as genetic relationship, phonology, or syntax, can either facilitate or impede the learning of a new language. In our current study, we aimed to address these gaps by analyzing standardized English assessment scores of English Language Learners (ELLs) from 46 international schools across 30 countries. We compared linguistic similarities between more than 40 L1s and examined how different linguistic structures of the L1 influenced the acquisition of L2, specifically in terms of genetic, phonological, and syntactic similarities. Our findings indicate that older ELLs learn English at a faster rate than younger ELLs, and among older ELLs, those who speak an L1 that shares linguistic similarities with English acquire the language faster than ELLs whose L1 is less closely related to English. These results highlight the transfer of linguistic knowledge from L1 to L2 and emphasize the importance of age of acquisition in L2 learning. Our study has important theoretical and pedagogical implications for research on second language acquisition.