This research investigated Spanish-English bilingual adults' initial learning of a third language (L3), Latin, comparing the learning processes and outcomes of early- and late-onset bilinguals. Thirty-three participants were classified as Early or Late Bilinguals according to their age of arrival to the USA, and they were introduced to Latin by means of an interactive computer programme that provided grammar explanation, input-based practice and explicit feedback. Results indicated that (1) overall, the two groups garnered similar benefits from the instructional treatment; (2) the Late Bilinguals maintained improvements in accurately marking noun case morphology in L3 production somewhat better than Early Bilinguals; and (3) there is a role for current age in explaining the variance observed in initial L3 learning, particularly when a target structure is morphologically complex and memory cannot be supported by existing L1/L2 knowledge. (Contains 4 tables, 2 figures and 12 notes.)