Much criminological research has used longitudinal data to assess change in offending over time. An important feature of some data sources is that they contain cross-sections of different aged individuals followed over successive time periods, thereby potentially conflating age and time. This article compares the substantive conclusions about the relationship between age and offending based on trajectory modeling analyses by time-of-assessment versus age-at-assessment. Analyses of a large sample of serious youthful offenders followed for 7 years consisted of estimation of group-based offending trajectories and multinomial logistic regressions examining how risk/protective factors distinguished between offending trajectories. Findings revealed that concordance using measurement approaches was strong at relatively high and low levels of antisocial behavior, with greater ambiguity between these two offending extremes. The relation of risk/protective factors to various trajectories was substantively similar across measurement approaches. (Contains 6 tables, 2 figures, and 8 notes.)