The Attitude Toward Own Aging Subscale (ATOA) is a frequently used measure of subjective aging. Although ATOA in midlife might assume a preparatory role for psychosocial adjustment in old age, research has been dominated by a focus on older adults. To enable a comparison of developmental trajectories of ATOA between middle-aged and young-old adults, we tested measurement invariance between age groups and over a 12-year study period. In addition, personality variables, health dimensions, and sociodemographic variables were investigated as predictors of developmental trajectories of ATOA. Data came from the Interdisciplinary Longitudinal Study of Adult Development (ILSE) with 2 birth cohorts (1930-1932: n = 500; 1950-1952: n = 501) followed over 12 years. Data analyses were conducted with confirmatory factor analysis for ordered-categorical variables and latent growth models. Support for the assumption of partial measurement invariance of ATOA was found in each age group, but not between age groups. Latent growth models revealed a steady decline in ATOA for young-old individuals, whereas ATOA trajectories in midlife were characterized by interindividual variation. Health variables predicted level of ATOA in the young-old. In midlife ATOA were shaped by a variety of factors. Future studies should be conducted with an awareness of differential item functioning of the ATOA scale across age groups. Furthermore, our results point to a greater modifiability of aging attitudes in middle-aged compared with young-old individuals, thus highlighting the importance of the midlife years in shaping developmental trajectories into old age.