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Music and language engage the dorsal auditory pathway, linked by the arcuate fasciculus (AF). Sustained practice in these activities can modify brain structure, depending on length of experience but also age of onset (AoO). To study the impact of early experience on brain structure we manually dissected the AF in bilinguals with and without music training (MT) who differed in the AoO of their second language (L2), or MT. We found the usual left-greater-than-right asymmetry in the volume of the long segment (LS) of the AF across all groups. However, simultaneous exposure to two languages from birth enhanced this leftward asymmetry, while early start of MT (=7) enhanced the right LS macrostructure, reducing the normative asymmetry. Thus, immersive exposure to an L2 in the first year of life can produce long-term plastic effects on the left LS, which is considered to be largely under genetic control, while deliberate music training in early childhood alters the right LS, whose structure appears more open to experience. These findings show that AoO of specific experience plays a key role in a complex gene-environment interaction model where normative brain maturation is differentially impacted by diverse intensive auditory-motor experiences at different points during development.