It has recently been suggested that bilingualism, rather than age of acquisition, is what underlies less than nativelike attainment in childhood L2 acquisition. Currently, however, the empirical evidence in favor of or against this interpretation remains scarce. The present study sets out to fill this gap, implementing a novel factorial design in which the variables age of acquisition and bilingualism have been fully crossed. Eighty speakers of Swedish, who were either L1 monolinguals, L1 simultaneous bilinguals, L2 sequential monolinguals (international adoptees), or L2 sequential bilinguals (childhood immigrants), were tested on phonetic, grammatical, and lexical measures. The results indicate consistent effects of age of acquisition, but only limited effects of bilingualism, on ultimate attainment. These findings thus show that age of acquisition – not bilingualism – is the primary determinant of L2 ultimate attainment.