The question of a sensitive period in language acquisition has been subject to extensive research and debate for more than half a century. While it has been well established that the ability to learn new languages declines in early years, the extent to which this outcome depends on biological maturation in contrast to previously acquired knowledge remains disputed. In the present study, we addressed this question by examining phonetic discriminatory abilities in early second language (L2) speakers of Swedish, who had either maintained their first language (L1) (immigrants) or had lost it (international adoptees), using native speaker controls. Through this design, we sought to disentangle the effects of the maturational state of the learner on L2 development from the effects of L1 interference: if additional language development is indeed constrained by an interfering L1, then adoptees should outperform immigrant speakers. The results of an auditory lexical decision task, in which fine vowel distinctions in Swedish had been modified, showed, however, no difference between the L2 groups. Instead, both L2 groups scored significantly lower than the native speaker group. The three groups did not differ in their ability to discriminate non-modified words. These findings demonstrate that L1 loss is not a crucial condition for successfully acquiring an L2, which in turn is taken as support for a maturational constraints view on L2 acquisition.