What people believe about their capacity to exert self-control (willpower), whether it is a limited or a nonlimited resource, affects their self-regulation and well-being. The present research investigated age-related differences in people’s beliefs—called implicit theories—about willpower. Study 1 (n = 802, age range 18–83 years) showed that with higher age people are more likely to believe that willpower is a nonlimited resource. Study 2 (n = 423) with younger (age 18–35 years) and older adults (age 60–98 years) replicated this finding and showed that age and a nonlimited willpower theory are related to perceived autonomy on demanding tasks (i.e., sense of self-determination), which might explain the age-related differences in willpower theories. Finally, experimental Studies 3a (n = 302) and 3b (n = 497) manipulated an autonomous mindset in younger (age 18–35 years) and older adults (age 60–87 years) and provided evidence for a causal effect of perceived autonomy on self-control-beliefs, supporting the proposed developmental mechanism. (PsycINFO Database Record (c) 2018 APA, all rights reserved)