This book provides an overview of research and thinking on age-related dimensions of language acquisition, intended for students, researchers, and educators with some experience in linguistics and applied linguistics. The first chapter introduces the variety of issues associated with age and language acquisition. Chapter 2 examines the evidence relating to speech milestones in the very early stages of acquisition (birth to 24 months), in the later stages, in speech processing, and in abnormal language development from language disorders. Chapter 3 reviews first-language-related evidence pertaining to the critical period hypothesis, and Chapter 4 reviews evidence related to the second language. The fifth chapter outlines and appraises the various explanations that have been offered for evidence of an age factor in language acquisition, including the aging of the senses, language interference or competition, neurolinguistics and the critical period, cognitive developmental views, affect and motivation, age and input, and the psychogrammar "decoupling" hypothesis. This chapter goes on to sketch the relationship between the age question and one particular model of language, Chomskyan nativism. Chapter 6 explores two major language issues in education linked to age: second language instruction at the elementary school level, and second languages for older adult learners. A substantial bibliography is included. (MSE)