While almost all observers agree that young children, older children, and adults differ both in initial rate of acquisition and in the levels of ultimate attainment typically achieved, they continue to disagree over whether the observed patterns are a function of nurture or nature. Is it simply that older starters "do not" do as well because they are less motivated, receive poorer quality input, spend less time on task, or (paradoxically) are hindered by superior cognitive development, or is it that they "cannot"? In particular, well-respected scholars differ on the existence, scope, and timing of putative maturational constraints on the human capacity for learning second (including foreign) languages. Some recent research on age differences is considered, in particular, studies purporting to provide evidence against claims of maturational constraints on (Second) Language Acquisition (SLA) and/or putative critical periods for L2 phonology, morphology and syntax. It is argued that, in each case, one or more of nine limitations or design flaws means that the findings are, in fact, unproblematic for at least some of those claims.