Many theories highlight the role social contexts play in shaping affective experience. However, little is known about how individuals’ social environments influence core affect on short time-scales (e.g., hours). Using experience sampling data from the iSAHIB, wherein 150 adults aged 18 to 89 years reported on 64,213 social interactions (average 6.92 per day, SD = 2.85) across 9 weeks of daily life, we examined how 4 features of individuals’ social ecology (between-person differences) and immediate social situations (within-person changes) were associated with core affect—valence and arousal—and how those associations differ with age. Results from multilevel models revealed that familiarity, importance, type of social partner, and gender composition of the social context were associated with affect valence and/or affect arousal. Higher familiarity, higher importance, and same-gender composition were associated with more positive affect valence and higher arousal. Interactions with family and friends were linked to more positive valence whereas nonfamily social partners were linked to higher arousal. Age moderated the associations between importance and affect arousal, and between type of social partner and both dimensions of core affect. Findings align with theoretical propositions, contributing to but also suggesting need for further precision regarding how development shapes the interplay between social context and moment-to-moment affective experience.