Although both concurrent and longitudinal relations between shyness and behavioral problems are well-established in childhood, there is relatively less work exploring these associations in emerging adulthood. In addition, age-related differences in the strength of these relations in child and adult samples have not been fully explored within the same study. We collected measures of shyness, internalizing and externalizing behaviors, and social problems in a sample of 94 typically developing 6-year-old children (50 female; Mage = 78.3 months, SD = 3.1 months) and 775 undergraduate students (633 female, Mage = 18.2 years, SD = 0.9 years) from parent-reported and self-reported questionnaires, respectively. Shyness interacted with age in predicting internalizing behaviors and social problems, but not externalizing behaviors. Specifically, shyness was concurrently and positively related to internalizing and social problems in young adulthood, but this relation was not found in childhood. Findings are discussed in terms of developmental consequences of shyness across the lifespan and limitations of relying on ratings from different informants when examining age-related differences.