Older adults report greater affective well-being in solitude than younger adults, but prior findings are based on correlational designs. We aim to examine age differences in affective well-being in solitude using an experimental design and to examine conflict de-escalation as a potential mechanism. In Study 1, 207 participants were randomly assigned to either a solitude or a social interaction condition. In Study 2, 128 participants were randomly assigned to one of four conditions: solitude-escalation, solitude-de-escalation, interaction-escalation, and interaction de-escalation. After a 15-min solitude or social interaction period, they reported their affective experiences. In Study 1, older (vs. younger) adults reported more positive affect overall. This age-related difference was greater in the solitude (vs. social interaction) condition; older adults reported less negative affect than younger adults in the solitude, but not the social interaction, condition. In Study 2, older (vs. younger) participants reported more high-arousal positive affect in the solitude-escalation condition. This difference was not significant in conflict de-escalation conditions. Our studies provide causal evidence of the relationship between solitude and affective well-being and advance our understanding of motivations that explain why older adults maintain better affective well-being in solitude.