This study examined the nature of the relationship between adolescents' subjective age (how old they feel) and chronological age, and explored whether dating, sex, and substance use predicted increases in adolescents' subjective age across a two-year period. The participants were 570 adolescents who were interviewed when they were first ages 12-19 and again two years later (ages 14-21). Results were that a quadratic function characterized the relationship between chronological and subjective age: subjective age increased from age 14 to 18.25 years, and then declined. Adolescents who dated, reported having sex, used alcohol, and used drugs at time 1 experienced an increased subjective age, that is, they felt older than less experienced adolescents by time 2. An older subjective age at time 1 did not predict increases in dating, sex, and substance use behaviors, with one exception: smoking. The results are consistent with theories assuming that self-perceptions of maturity may arise from engaging in "adult-like" behaviors. (Contains 1 figure and 5 tables.)