Understanding persistence and changes in prosociality across the life span is fundamental to inform theory and practice. As life expectancy increases and pressing societal challenges demand increasing generosity and cooperation among individuals, it is crucial to understand intergenerational interactions. We present the findings from a novel lab-in-the-field experiment (N = 359, 18-90 years) that examines generosity and cooperation between generations. Our methodological approach allows us to study the effect of age on prosocial behavior as a function of the age of an unknown partner. We ask participants to make several decisions, and to state their expectations for their partners' behavior, in a dictator game and a prisoner's dilemma game with real monetary outcomes. The dictator game serves as a measure of generosity, whereas the prisoner's dilemma serves as a measure of cooperation. We find that individuals used age as key information to condition behavior. Generosity was greater among older adults in response to young and older relative to middle-aged partners. Among younger adults, cooperation was greater in response to middle-aged and older partners relative to their own age cohort. All age groups expect less cooperation from young partners than from older and middle-aged partners. However, relative to young adults, older adults are more cooperative with young partners. Our study has crucial implications for the understanding of human generosity and cooperation across the life span. (PsycInfo Database Record (c) 2021 APA, all rights reserved).