This study examines age differences and developmental changes in resistance to peer influence using a new self-report instrument that separates susceptibility to peer pressure from willingness to engage in antisocial activity. Data from four diverse samples of over 3,600 individuals aged 10 to 30 were analyzed. The results show that resistance to peer influence increases linearly between ages 14 and 18, with little evidence of growth between 10 and 14 or between 18 and 30. This pattern is consistent across all demographic groups, including gender, ethnicity, and socioeconomic status. The findings suggest that middle adolescence is a significant period for developing the capacity to resist peer pressure, particularly in terms of standing up for one's beliefs. The study also highlights gender differences, with girls generally showing higher resistance to peer influence than boys, and ethnic differences, with African-American individuals reporting greater resistance than other ethnic groups. Socioeconomic status affects resistance to peer influence in longitudinal analyses but not in cross-sectional analyses. The study extends the understanding of adolescent psychosocial development by providing a more nuanced view of age differences in susceptibility to peer pressure.This study examines age differences and developmental changes in resistance to peer influence using a new self-report instrument that separates susceptibility to peer pressure from willingness to engage in antisocial activity. Data from four diverse samples of over 3,600 individuals aged 10 to 30 were analyzed. The results show that resistance to peer influence increases linearly between ages 14 and 18, with little evidence of growth between 10 and 14 or between 18 and 30. This pattern is consistent across all demographic groups, including gender, ethnicity, and socioeconomic status. The findings suggest that middle adolescence is a significant period for developing the capacity to resist peer pressure, particularly in terms of standing up for one's beliefs. The study also highlights gender differences, with girls generally showing higher resistance to peer influence than boys, and ethnic differences, with African-American individuals reporting greater resistance than other ethnic groups. Socioeconomic status affects resistance to peer influence in longitudinal analyses but not in cross-sectional analyses. The study extends the understanding of adolescent psychosocial development by providing a more nuanced view of age differences in susceptibility to peer pressure.