The proposition that "21 is the new 18" has migrated from popular commentary into serious policy argument, invoked in debates over the age of majority, criminal responsibility, and the allocation of adult rights. This paper subjects the proposition to systematic examination by disaggregating it into four separable claims: a demographic claim about the timing of role transitions, a developmental claim about the maturation of capacity, a subjective claim about self-perceived adulthood, and a normative claim about where legal thresholds ought to sit. Drawing on nationally representative demographic data, cross-national household formation statistics, meta-analytic and multinational developmental research, and large-scale skills assessments, the paper finds that the four claims are supported to very different degrees. The demographic claim is well supported but poorly calibrated: role transitions have been displaced, yet by a magnitude closer to five or seven years than to three, and unevenly across markers. The developmental claim is not supported: no body of evidence demonstrates that maturational timing has changed across cohorts, and the neuroscientific literature routinely enlisted in its support is cross-sectional rather than cohort-comparative. The subjective claim is supported, though the survey evidence circulating in public discourse is frequently misreported; two widely repeated statistics are shown here to invert or conflate their sources. The normative claim does not follow from the others, because the evidence identifies constrained circumstance rather than diminished capacity as the operative mechanism. The paper concludes that 21 is not the new 18 in any developmental sense, that it understates a real sociological displacement, and that raising the age of majority would misdiagnose a problem of political economy as one of arrested development.