Children acquiring their first language complete the feat within a biological window of four to six years of age. By contrast, the ages at which different L2 learners may begin learning the new language range wildly. Thus, age emerges as a remarkable site of difference between L2 and L1 acquisition. Perhaps for this reason, understanding the relationship between age and L2 acquisition has been a central goal since the inception of the field of SLA. Two issues are hotly debated. One pertains to the possibility that a biological schedule may operate, after which the processes and outcomes of L2 acquisition are fundamentally and irreversibly changed. This is also known as the Critical Period Hypothesis in L2 learning. The other issue relates to the possibility that there may be a ceiling to L2 learning, in the sense that it may be impossible to develop levels of L2 competence that are isomorphic to the competence all humans possess in their own mother tongue. Although the topic of age has been investigated profusely in SLA, clear or simple answers to vital questions about the relationship between age and L2 learning have not been easy to produce. As you will see in this chapter, the accrued findings remain difficult to reconcile and interpret, and many questions to understand universal age effects on L2 acquisition remain open.