This thesis investigates the influence of age of acquisition on the long-term second language development of international adoptees. Because age of acquisition typically coincides with the onset of bilingualism, the study of maturational age effects in second language acquisition has been empirically and conceptually entangled with changes in language input and use. For international adoptees, however, because the adoptive language is acquired under similar linguistic conditions as non-adopted peers – albeit at a later age of acquisition – questions of age and second language acquisition can be investigated without confounding influences of bilingualism. Study I presents the theoretical argument that, because of the delay in acquisition, the language development of international adoptees should be regarded as a special case of second language acquisition. Furthermore, consistent with the contemporary study of second language acquisition, the effects of this delay should be investigated through ultimate attainment observed in adults. Study II shows that adults in Sweden who had been adopted from Spanish-speaking countries, and Spanish-Swedish bilinguals with the same age of acquisition (3-8 years), have greater difficulty in perceiving Swedish vowel distinctions that do not exist in Spanish compared to native Swedish speakers. This suggests that age of acquisition is a decisive factor for speech perception in a second language. In Study III, Chinese-Swedish adoptees are found to deviate from native Swedish speakers in their production of Swedish vowels that are phonologically identical in Chinese, but not in vowels that are distinctive in both languages. While these results are consistent with predictions based on assumptions of transfer and interference in bilingual speakers, they cannot be explained based on these premises. Instead, the results suggest that early language-specific experiences will affect the pronunciation of vowels in the second language regardless of whether the native language is in use or not. In Study IV, the neural underpinnings of the behavioral results are investigated electrophysiologically, using EEG. This study shows that adult adoptees retain increased neural sensitivity to a native Chinese lexical tone contrast without any exposure to the language for over 15 years. This is reflected in a fast neural response stemming from the auditory cortex and is indexed by the mismatch negativity event-related potential. This suggests that native language sensitivity is not only retained, but is continuously involved in the moment-to-moment processing of speech sounds. Neural oscillations furthermore reveal the involvement of inhibitory processes to attenuate this sensitivity. Finally, positive correlations between neural responses to the native and the adoptive language show that native language retention is not in itself an impediment for second language acquisition. The results from these three studies show how language-specific experiences lead to irreversible specialization in the brain, which will affect the long-term acquisition of a second language. This finding invites a re-evaluation of the hypothesis of a critical period for second language acquisition, based on the notions of probabilistic epigenesis and flexible behavioral adaptation following experience-based functional neural reorganization in early childhood.