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.