Policy loves round numbers. Biology does not. The neatest myth in modern drug debates is that brains “finish” at 25. It’s a tidy line for lawmakers, a mantra for public campaigns — and, as two substance-use researchers argue, a fiction that risks bad policy.
Bryon Adinoff, an addiction psychiatrist at the University of Colorado, and Julio Nunes, a Yale psychiatry resident, recently published a perspective in the American Journal of Drug and Alcohol Abuse questioning the science behind the 25-year cutoff. Their case lands right as new brain-mapping work shows lifespan turning points that don’t respect our legal birthdays. Together, the message is blunt: if you need a single age to make rules about cannabis, alcohol, nicotine, or psychedelics, neuroscience won’t give it to you.
“Brain development is continuous and individual specific. There is no single biological cutoff age that determines safe or unsafe substance use.” — from Adinoff and Nunes’s perspective in AJDAA
Their piece — Challenging the 25-year-old ‘mature brain’ mythology — is not an anything-goes manifesto. It’s a plea to stop building policy on a number that never had a solid empirical footing.
How a tidy myth took root
The 25 idea came from a loose reading of imaging studies showing that certain prefrontal systems, which support planning and impulse control, keep maturing into the early-to-mid 20s. That part is true. The leap was the rest: that maturation “finishes” on schedule, and that finishing equals a sharp drop in risk.
The science is messier. Different regions mature at different tempos, and development doesn’t march in straight lines. A major study in Nature Communications analyzed structural brain networks from birth to age 90 and found four turning points at roughly 9, 32, 66, and 83. Integration and segregation of brain networks rise and fall across these epochs. At about 32, for example, measures of global efficiency peak and then begin to decline while modularity trends upward — hardly a cliff at 25.
“Major turning points occur around nine, 32, 66, and 83 years old.” — Nature Communications (2025) lifespan connectome study
That variability extends between people, too. Sex-linked developmental timing differences exist as overlapping tendencies, not binary “male/female brains,” as reviewed in a multimodal imaging overview (PubMed). Genes, hormones, environment, education, and health conditions all interact. In short: maturation is a curve, not a cliff, and your curve may not be mine.
So where does 25 come from? From a search for a bright line that the data never promised. A line is convenient for enforcement and messaging. But convenience is not evidence.
What better policy looks like
If the brain doesn’t hand us a “correct” minimum age, what does? Adinoff and Nunes argue for a broader frame: fold real-world harms, feasibility, equity, and lived experience into the decision. Epidemiology matters. So do enforcement consequences. Criminalization, after all, has reliably produced incarceration and disparities without clear, lasting reductions in overall use — even as reform expands access to treatment and harm-reduction services.
That shift is already underway. Instead of anchoring to 25, focus on patterns of use and contexts of risk. Clinicians don’t diagnose addiction by a cannabinoid percentage or a birthday; they look for behaviors and consequences. The DSM-5 definition of substance use disorder centers on impaired control, social impairment, risky use, and pharmacologic indicators (APA overview).
Everyday users can translate that into a quick self-check. The researchers suggested a “5 Cs” shorthand for early warning signs:
- Consequences: problems with sleep, mood, work/school, money, or relationships tied to use
- Cravings: stronger urges than you expected
- Continued use despite harm: you see the damage but keep going
- Compulsion or loss of control: more than planned, unable to cut back
- Coping: using becomes your main way to manage stress or emotions
That lens is substance-agnostic. A family history of psychosis elevates risk from high-THC cannabis; a cardiac condition changes the calculus for stimulants, including caffeine. Psychedelics may help some in clinical settings and destabilize others, particularly without screening or support. Who you are, how you use, and the setting often matter more than a birthday.
Zoom out, and the social fabric is part of the risk environment. Many cultures have long used psychoactives in supervised rites of passage: guided, ritualized, embedded in community norms. The benefits there often flow from structure and meaning, not the molecule alone. Harm reduction borrows that insight for modern life: meet people where they are, offer accurate information, and reduce avoidable harms. Sheila Vakharia’s The Harm Reduction Gap is a succinct primer on why this public-health approach saves lives (book).
None of this means abandon age limits. Societies set them for many reasons: developmental averages, social expectations, enforcement practicality. It does mean be honest about their basis, and don’t invoke neuroscience to defend numbers neuroscience can’t justify. If lawmakers prefer 21 to 18 for alcohol or cannabis, they should say it reflects a policy balance — not a neural finish line at 25.
Evidence can still inform the details:
- Graduated, lower-risk access models: limit high-potency products or marketing to younger adults while allowing legal, regulated access to reduce illicit-market harms
- Context-first education: teach dosing, mixing risks, and safer-use practices instead of fear scripts, particularly for college-age populations
- Screening and support: integrate brief interventions into primary care; make treatment easy to access and stigma-light via healthcare and community settings
- Equity by design: avoid policies that widen racial or socioeconomic disparities in stops, searches, or sentencing
On communication, retire the myth cleanly. Say what the science supports. Adinoff and Nunes offer more precise language: “brain development is continuous and individual specific,” “different brain regions mature at different rates,” and “there is no single biological cutoff age that determines safe or unsafe substance use.” Clear, accurate, and far harder to put on a billboard — which is precisely the point.
The irony is that the truer story might be more compelling. Lifespan science reveals a brain that reorganizes with purpose, trading integration and specialization in different seasons of life. The Nature Communications team mapped those seasons: a childhood pivot near 9, a striking topological shift around 32, slowing change through midlife, and late-life reconfiguration. The lines we draw in civil code can learn from that complexity, even if they’ll never match it.
Policy thrives on clarity; biology on nuance. We should build rules that can hold both. The first step is easy: stop pretending a messy, beautiful organ obeys a round-number myth.