This article explores how peers influence the process of adult identity development during the transition to adulthood. The influence of peers leads to similar individuals adopting differing definitions of adulthood. Utilizing data from interviews with 60 young adults who are all exactly 30 years old, findings indicate that peer groups can partly explain variation in self-perceived definitions of adulthood. Respondents described how peers influence the transition to adulthood in two ways. First, they measure the timing of their transitions relative to their peers. Second, they assess the nature of their transitions relative to what they perceive to be the normative nature of that transition within their peer group. While this process was reported by respondents across gender and education level, the outcomes varied between individuals who are demographically similar. Variation in self-perceived status is due in part to the differences between peer groups, as the reference point for each individual varies from one peer group to another. These findings suggest that norms about adulthood are perceived at the group level, which can explain why differing feelings of adulthood exist among individuals who have completed comparable transitions and share similar status characteristics. As previous research has focused on adulthood norms that exist primarily at the societal level, this study expands on that work by suggesting that salient adulthood norms may be developed and referenced at multiple levels.