The present study examined age differences in young people's knowledge of legal concepts presumed to be important in terms of the capacity to instruct legal counsel. Forty-eight subjects in each of grades 5, 7, and 9 together with 48 young adults participated in a structured interview containing four scenarios, each depicting a young person who had committed a criminal offense, was charged, and retained a lawyer. Subjects were questioned regarding their knowledge of the role of defense counsel, lawyer-client confidentiality, the meanings of "plead Guilty" and "plead Not Guilty," and what happens during a trial. Results revealed that some aspects of legal knowledge were understood adequately by subjects at all ages (for example, definitions of defense counsel's role, plea of Guilty) while subjects were uniformly ignorant of others (for example, the meaning of "plead Not Guilty"). Subjects' responses to most of the questions improved with age (for example, understanding of lawyer-client confidentiality and trial descriptions). Certain misconceptions also increased, rather than diminished, with age (for example, meaning of a Not Guilty plea). Findings are discussed in terms of both developmental theory and practical implications for the legal system. 18 references, 3 notes, and 5 tables (Author abstract)