This longitudinal study assessed the relative importance of second language exposure, motivational and attitudinal factors, and executive functions in predicting receptive vocabulary acquisition in children and adolescents after controlling for background characteristics (socio-economic status, non-verbal intelligence) and prior knowledge. The sample comprised 721 participants from French-speaking Belgium: 369 fifth-graders from 13 elementary schools and 352 eleventh-graders from 9 secondary schools. The background, input, and socio-affective data were obtained through questionnaires, the cognitive and language data through a round of computerized data collection, twice with an 18-month interval. While some input and motivational variables were predictive of vocabulary acquisition when analyzed separately, most of them lost significance once combined. Measures of executive functions were not related to vocabulary acquisition. Classroom input was predictive among children and adolescents, but other age-related differences emerged. The findings call for caution regarding the separate treatment of factors believed to be involved in second-language acquisition.