Across a variety of decision domains, older adults desire fewer options than younger adults, but underlying mechanisms of these age differences remain unknown. Three studies (total N = 586) were conducted to investigate the extent to which age differences in choice set size preferences are driven by decision-making self-efficacy, maximizing, motivational priorities, and other theoretically implicated covariates. First, a large-scale survey (Study 1) examined the age trajectory of, and underlying influences on, choice set size preferences in a life-span sample. Subsequent laboratory-based experiments (Studies 2a-2c) manipulated self-efficacy, maximizing, and motivational priorities, respectively, among younger adults and measured effects on choice set size preferences and information search. Finally, a laboratory-based quasi-experimental design (Study 3) assessed the effect of experimentally manipulated self-efficacy on older versus younger adults' decision making. Combined results suggest that age differences in choice set size preferences are linear, gradual, and relatively domain-general. They are not mediated by any of the variables tested, with the exception of vocabulary scores, which accounted for 1.5% of variance in choice set size preferences above and beyond age. However, inter-individual differences, if not age differences, in choice set size preferences and information seeking may be amenable to altering via manipulations of decision-making self-efficacy. Implications for research on aging and decision-making, as well as public policy, are discussed.