Earlier studies of maturation effects and ultimate achievement – UA, eventual knowledge of a second language (L2) – mainly involved immigrants to English-speaking countries tested on the native-likeness (how much they sounded like native speakers) of their L2 phonology or morphosyntax. In these classic studies, a consistent finding was that perceived native-likeness inversely correlated with increasing age of onset of acquisition (AoA), a finding linked to an optimal period for SLA. The past decade has developed new perspectives on these issues, eschewing the notion of native-likeness as a gold standard, incorporating numerous factors in addition to AoA and looking at the interaction of the two languages of the bilingual. Factors include environment of acquisition, input and individual learner characteristics. This chapter outlines the classic studies of AoA and the Critical Period Hypothesis (CPH) before turning to UA in phonology, morphosyntax and processing. It then reevaluates the term native speaker, a label that has received criticism for perpetuating Eurocentric models; it finally recapitulates several components that can be taken into account to explain the range of ultimate achievement in child and adult L2.