This monograph presents information to help educators consider how to determine the best age for each child to begin his or her formal schooling. Trends in state requirements for school entrance age are described, and research in areas such as mental age, reading readiness, child development, screening instruments, and experimental preschool programs is reviewed. Educational approaches that attempt to address the problem of differential readiness in young children are also discussed. After a brief introduction, the first section discusses the implications of current entrance age trends. This is followed by an extended discussion of issues and trends that have influenced school entrance age: the relationship of chronological age and mental age to reading readiness; the child development concept; the early intervention approach; changes in kindergarten programs; developmental screening for school placement; and issues associated with being the youngest in the class. The next major section addresses alternative approaches to differential readiness in children. These include full-day kindergartens, four-year-old kindergartens, transition programs, early admission and delayed entrance to school, retention, teacher effectiveness, and other approaches to the problem of failing pupils. A summary chapter reviews five historical stages in school practice regarding entrance age interests and concerns, and presents a comprehensive set of implications and recommendations for educators and policy-makers derived from the study as a whole. A bibliography is included. (TE)