This paper lays out a principle of consistency and shows how it justifies lenient sentences for juvenile offenders. The principle holds that, if the state disadvantages someone by applying the rationale of a legal rule, it has a reason of fairness to act on that same rationale when doing so would advantage them. As applied to juvenile leniency, the law systematically assumes that juveniles are diminished in their normative capacities. These rules, putting the penal context to one side, operate to disadvantage those juveniles for whom the assumption is false. This places the state under a duty, owed to these juveniles as a matter of fairness, to act on the same assumption when it would work to their advantage. Such juveniles are therefore owed lenient sentences.