The antagonistic pleiotropy (AP) mechanism of aging was first proposed by G.C. Williams in 1957. However, practical application of this theory for the targeted search of longevity interventions has lagged, owing to a lack of clear understanding of the conditions under which the same gene or trait may have opposite effects on fitness in young versus old organisms. We propose that changes in the somatic environment may result from allometric growth, physiological differences between adult and juvenile life stages beyond those caused by aging, and ontogenetic niche shifts, and we provide well-documented examples of AP mechanisms corresponding to these conditions. We then test this understanding through testable predictions. Specifically, we demonstrate that (1) traits that have diverged the most between developmental stages contribute the most to aging; (2) organisms with negligible senescence exhibit minimal differences between adult and juvenile life stages; and (3) among taxonomically close organisms, stronger differences between adult and juvenile stages are associated with higher aging rates, while greater similarity is associated with lower aging rates. This understanding opens opportunities for the targeted identification of AP mechanisms based on the analysis of organisms' developmental trajectories. Additionally, it suggests two potential approaches for mitigating AP: suppression of adverse genes or traits in late life, or prevention or reversal of alterations in the somatic environment that convert previously beneficial traits into detrimental ones.