Bryon Adinoff, MD, says that when he “kept hearing this concern about using cannabis before the age of 25, because the brain is still developing,” he was puzzled.
Adinoff – a clinical professor of psychiatry in the University of Colorado Anschutz School of Medicine – has deep experience with this field. He’s been a board-certified addiction psychiatrist for decades, he ran substance-use disorder programs at the U.S. Department of Veterans Affairs, and he’s editor-in-chief of The American Journal of Drug and Alcohol Abuse.
“I’m very familiar with the literature,” he says. “I’ve been around since neuroimaging started. I’ve kept abreast of developments, particularly in kids and adolescents, and I never remembered seeing anything” providing evidence that the brain isn’t fully developed until age 25.
Yet that claim is often repeated on various academic and government websites, Adinoff says. Some medical organizations have advocated for raising the minimum legal age for recreational cannabis sales to 25, citing the supposed brain-development milestone. Currently, the minimum age to buy recreational cannabis is 21 in the 24 states that permit sales.
In 2025, the Colorado legislature considered a measure to raise the legal age for buying higher-potency recreational marijuana products from 21 to 26, with one advocacy group claiming that’s the age when young people’s brains reach full development. The bill eventually died in committee.
So Adinoff went digging. “I started looking it up and I couldn’t find anything” proving there’s an age-25 finish line for brain development, he says. He asked an AI chatbot, “when does brain development end?” The answer was 25. “So I asked, ‘Where is that coming from?’ It gave me a bunch of citations. I checked, and none of those citations say that. I started asking colleagues, and even random people, when the brain stops developing, and everybody – scientists, laymen – would say, ‘25 years.’ It was fascinating.”
Adinoff began to see the age-25 threshold as a myth masquerading as science – a catchy claim that gets repeated so often that it becomes broadly accepted as fact, when in fact there’s no solid science behind it.
That’s the argument he and Julio C. Nunes, MD, of the Yale School of Medicine make in a perspective article, “Challenging the 25-year-old ‘mature brain’ mythology: implications for the minimum legal age for non-medical cannabis use,” published recently in the journal Adinoff edits. The article was independently reviewed, Adinoff says.
“I felt compelled to write this,” he says. “I mean, somebody had to.”
Age 25 not a finish line
Adinoff and Nunes reviewed the neuroscience literature and found that no study actually identifies age 25 as a meaningful developmental "finish line." They say that about 90% of brain development is completed by the age of 5, and that the most important functions – such as impulse control and emotional regulation – largely mature by ages 18 to 21.
Some subtle changes – such as synaptic pruning, a process in which the brain improves neural signaling efficiency – do continue into the late 20s, the paper says, but these processes are gradual and there's no special biological threshold at age 25.
“I think it’s just a catchy number,” Adinoff says with a grin.
As for whether cannabis harms people between the ages of 18 and 25 more than older adults, the article says: “Available data do not show that cannabis use between ages 21 and 25 confers uniquely severe or irreversible harm compared to use beginning after 25.”
The article also argues that raising the minimum age for cannabis sales above 21 could have unwelcome consequences. They include possibly driving younger cannabis users to illegal drug markets selling potentially unsafe products; disproportionately criminalizing young people, especially from marginalized communities; undermining trust in law enforcement and public health institutions; and stripping adults of personal autonomy in making their own health decisions, the authors say.
The bottom line, Adinoff says, is that “a myth can arise even in the scientific community, with no empirical basis. It can persist and envelop society if no one looks into it or challenges it.”
A disconnect
Adinoff is president of Doctors for Drug Policy Reform, a national nonprofit group that advocates for equitable, science-based substance-use policies. He notes that Americans can vote, sign a contract, go into the military, and get married at age 18, and yet aren’t allowed to buy alcohol or tobacco until age 21.
“I’m not making a policy judgment one way or the other on that, but there’s a disconnect in terms of personal agency and biological development that we need to be mindful of,” he says.
Adinoff says he’s not arguing that cannabis use is completely harmless at any age. “It’s certainly feasible that cannabis, and high-potency cannabis in particular, may have harmful effects. I think the jury is still out on long-term effects, like schizophrenia. I’ve not found anything convincing one way or the other, but those are very important questions that need to be investigated.”
Adinoff believes that the push to raise the age limit for cannabis sales is largely “a trope for people who want to reverse cannabis legalization but realize that’s not going to happen. They know the greatest use of cannabis is between the ages of 18 and 25. I feel there were very political reasons why this started and why this mythology persists.”
Challenging common assumptions “is a temperament thing I was born with,” Adinoff says. “Over the years, a few of my papers have asked, ‘What’s going on here?’ I think those have been some of my most successful papers, and the ones I’m most proud of.”
