Few researchers would doubt that ultimate attainment in second language grammar is negatively correlated with age of acquisition, but considerable controversy remains about the nature of this relationship: the exact shape of the age-attainment function and its interpretation. This article presents two parallel studies with native speakers of Russian: one on the acquisition of English as a second language in North America (n = 76), and one on the acquisition of Hebrew as a second language in Israel (n = 64). Despite the very different nature of the languages being learned, the two studies show very similar results. When age at testing is partialed out, the data reveal a steep decline in the learning of grammar before age 18 in both groups, followed by an essentially horizontal slope until age 40. This is interpreted as evidence in favor of the critical period. Both groups show a significant correlation between ultimate attainment and verbal aptitude for the adult learners, but not for the early learners. This is interpreted as further evidence that the learning processes in childhood and adulthood not only yield different levels of proficiency but are also different in nature.Age effects in (second) language learning are widely acknowledged, but their exact nature remains controversial, in particular, the concept of a critical period for second language acquisition (SLA). In about the last 15 years, numerous arguments against the critical period hypothesis (CPH) have been formulated: a few studies have failed to find a clear correlation between age of acquisition and ultimate attainment; many more researchers accept the negative correlation as a fact, but they argue that it is attributable to a confound between age of acquisition and one or more other variables, such as length of residence, age at testing, the nature of the input received as a function of age, the extent to which education was provided in the second language (L2) or the first language (L1), the (lack of) motivation to integrate fully with the L2 society, or simply the amount of practice

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