Individuals’ implicit theories of intelligence exist on a spectrum, from believing intelligence is fixed and unchangeable, to believing it is malleable and can be improved with effort. A belief in malleable intelligence leads to adaptive responses to challenge and higher achievement. However, surprisingly little is known about the development of academic-domain-specific theories of intelligence (i.e., math vs. reading and writing). The authors examined this in a cross-section of students from 1st grade to college (N = 523). They also examined whether students hold different beliefs about the role of fixed ability in adult jobs versus their own grade. The authors’ adult-specific beliefs hypothesis states that when children learn societally held beliefs from adults, they first apply these beliefs specifically to adults and later to students their own age. Consistent with this, even the youngest students (1st and 2nd graders) believed that success in an adult job requires more fixed ability in math than reading and writing. However, when asked about students in their own grade, only high school and college students reported that math involves more fixed ability than reading and writing. High school and college students’ math-specific theories of intelligence were related to their motivation and achievement in math, controlling for reading and writing-specific theories. Reading and writing-specific theories did not predict reading and writing-specific motivations or achievement, perhaps because students perceive reading and writing as less challenging than math. In summary, academic-domain-specific theories of intelligence develop early but may not become self-relevant until adolescence, and math-specific beliefs may be especially important targets for intervention.