Decision-making competence is the ability to follow normative principles when making decisions. In a longitudinal analysis, we examine the robustness of decision-making competence over time, as measured by two batteries of paper-and-pencil tasks. Participants completed the youth version (Y-DMC) at age 19 and/or the adult version (A-DMC) eleven years later at age 30, as part of a larger longitudinal study. Both measures are comprised of tasks adapted from ones used in experimental studies of decision-making skills. Results supported the robustness of these measures and the usefulness of the construct. Response patterns for Y-DMC were similar to those observed with a smaller initial sample drawn from the same population. Response patterns for A-DMC were similar to those observed with an earlier community sample. Y-DMC and A-DMC were significantly correlated, for participants who completed both measures, 11 years apart, even after controlling for measures of cognitive ability. Nomological validity was observed in correlations of scores on both tests with measures of cognitive ability, cognitive style, and environmental factors with predicted relationships to decision-making competence, including household SES, neighborhood disadvantage, and paternal substance abuse. Higher Y-DMC and A-DMC scores were also associated with lower rates of potentially risky and antisocial behaviors, including adolescent delinquency, cannabis use, and early sexual behavior. Thus, the Y-DMC and A-DMC measures appear to capture a relatively stable, measurable construct that increases with supportive environmental factors and is associated with constructive behaviors.