The Massachusetts Supreme Judicial Court’s 2024 decision in Commonwealth v. Mattis marked the first time any state has categorically banned life without parole beyond the juvenile context, applying that ban to individuals aged eighteen to twenty. In doing so, the court recognized that these individuals—known as “emerging adults”—share key developmental traits with juveniles, including heightened impulsivity, greater susceptibility to peer influence, and a diminished capacity to assess long-term consequences. These developmental differences make emerging adults more similar to juveniles than to fully mature adults, undermining the justification for life without parole—the harshest punishment available short of the death penalty.
This Note argues that the reasoning underlying the Mattis decision does not end at age twenty. Developmental science generally recognizes “emerging adulthood” as extending through age twenty-five, and the characteristics identified in Mattis persist throughout that period, undermining any meaningful distinction at twenty-one. The age-crime curve likewise shows that criminal behavior peaks in late adolescence and early adulthood before declining sharply, while recidivism data further challenges the assumption that individuals who commit serious offenses during this period remain permanently dangerous. Together, this evidence weakens retributive and deterrence-based justifications for life without parole and supports sentencing approaches that preserve the possibility of rehabilitation.
Situating Mattis within a broader national context, this Note argues that the decision reflects—and accelerates—a shift toward development-informed sentencing. Courts, legislatures, and prosecutors increasingly recognize emerging adults as a distinct category warranting individualized and rehabilitative approaches. Building on these developments, this Note contributes to the growing legal discourse on emerging adulthood by showing how Mattis provides a framework for extending categorical protections through age twenty-five, aligning sentencing law with developmental science, principles of proportionality, and modern understandings of culpability.
