The present research was conducted to map the hierarchical structure of youths' personality traits, to identify the foundational level of this structure, and to test whether the meanings of some youth personality dimensions shift with age. We addressed these issues by analyzing personality parent reports describing a cross-sectional sample of 16,000 children, adolescents, and young adults (ages 3 to 20). These parent reports were made using a broadband measure of youths' personal characteristics, the common-language California Child Q-Set. Analyses of the full sample and comparisons of 16 age groups supported three main conclusions. First, the hierarchical structure of youths' personality traits both resembles and differs from the adult personality hierarchy in important ways. Second, a set of six dimensions—Extraversion, Agreeableness, Conscientiousness, Neuroticism, Openness to Experience, and Activity—may constitute the foundational level of the youth personality hierarchy from middle childhood through adolescence. This “Little Six” structure represents a union of the most prominent personality and temperament dimensions. Third, the meanings of some youth personality dimensions (e.g., Activity, Conscientiousness) shift systematically with age. These findings advance our understanding of when and how personality structure develops during the first two decades of life.