"The Teen Brain Isn't Done Until 25" Is a Script, Not a Finding

You have probably heard it said with a shrug, in a pickup line or on the sidelines of a game: "Well, the teen brain isn't done until 25." It usually follows a forgotten assignment, a reckless choice or a sharp comment at dinner. It sounds like science. It also works like a sentence that ends the conversation.

I understand why parents reach for it. It explains behavior that frustrates us and gives us a reason to be patient. But when I looked at what the researchers who study adolescent development actually report, I found something more precise and, for parents, far more useful. The brain does not finish on one birthday. Different abilities settle at different ages, and some of them settle well before the slogan suggests.

Three Measures, Three Different Ages

Start with thinking skills. A University of Pittsburgh team led by Brenden Tervo-Clemmens and Beatriz Luna pooled data from 10,766 people aged 8 to 35, drawn from four U.S. datasets and 23 measures of executive function: the skills behind holding information in mind, shifting attention and stopping yourself before you act. In their 2023 study in Nature Communications, those skills developed rapidly between about 10 and 15 and then stabilized at adult levels around 18 to 20.

Now look at a different measure. Laurence Steinberg of Temple University and an international team compared more than 5,000 people aged 10 to 30 in 11 countries. In their cross-national study in Developmental Science, sensation seeking peaked around 19, while self-regulation kept rising and reached a plateau between 23 and 26. That is where the mid-20s idea finds real support. It is also a comparison of age groups at one point in time, not the same people followed over the years.

Then there is context. BJ Casey, now at Barnard College, has long argued that young people can show adult-level self-control when things are calm, but that it is less reliable under pressure. In a 2016 study in Psychological Science of 110 people aged 13 to 25, led by Alexandra Cohen with Casey as senior author, the team found that under negative emotional arousal, 18 to 21 year olds performed worse on a self-control task than adults over 21.

So which is it: 18, 20 or 25? The fair answer is that it depends on what you measure. Other scientists point out that structural brain scans keep changing past 20. The dual-systems model that Steinberg and Casey helped build, in which a fast reward drive outpaces slower control, is influential. Jennifer Pfeifer of the University of Oregon and Nicholas Allen of the University of Melbourne argued in a 2012 paper in Trends in Cognitive Sciences that it is too simple and underplays social context. Every one of these findings describes groups and averages. None of them can tell you what your own 16-year-old is ready for.

What a Slogan Does at Home

Research describes. A slogan instructs. When "not done until 25" moves from a lab finding to a household rule, it quietly changes how we treat young people. It becomes a reason to decide for them, to hold back responsibility, to wait.

Waiting has a cost. Judgment does not arrive fully formed in the mid-20s. It is earned in reps: making a call, living with the result, adjusting, trying again. If we tell a teenager their judgment can't be trusted for another decade, we remove the very practice that builds it. Then we act surprised when a 24-year-old shows up at a first job unsure of what they believe or how to stand behind it.

Look closely at what Casey's finding points to. The gap appeared under emotional pressure. That is not a reason to keep young people away from hard moments. It is a reason to let them practice hard moments while someone who loves them is still close enough to help them make sense of what happened.

Many of us were raised on our own version of this message. Wait your turn. Earn the right to an opinion. Let the adults decide. I wrote about that inheritance in Parents Were Handed the Same Script. The "25" line can become the newest edition of an old instruction: you are not ready to be yourself yet.

Match the Support to the Skill, Not the Birthday

None of this means handing a 15-year-old the car keys and hoping for the best. Luna's work is careful on this point: the 18 to 20 plateau describes lab tasks of thinking control, not emotional or social maturity. What it does mean is that support should fit the skill in front of you rather than a single number.

Where thinking skills are strong, give real decisions. A teenager who can plan, juggle several moving parts and pause before acting can own a budget, a weekly schedule or a piece of a family project. Let the plan be theirs, including the parts you would have done differently.

Where emotion runs high, stay near. A rejection, a hard grade, a heated group chat: these are the moments when young people are still building control. Instead of stepping in to decide, help them slow the moment down. Ask what they want to do once the feeling passes, and then let them do it.

Where the pull toward intensity is strong, give it somewhere to go. Steinberg's newer collaborative work takes this seriously: a 2025 nine-country study led by Natasha Duell, with Duke's Jennifer Lansford among its authors, built a way to measure positive risk taking, such as trying new things or speaking up. A teen who craves a rush can find it in an audition, a tryout, a business idea or a role nobody expected them to want.

And listen to your own language. "Your brain isn't done" tells a young person to wait. "You're building this right now" tells them their choices count. The adults who help most are the ones who look for what a young person can already do, the idea at the heart of The Best Mentors Discover You.

The research gives us a map with several finish lines, not one. Treating the farthest one as the only one holds back the young person standing in front of you, who is ready for more today than any slogan admits.

Research cited

Tervo-Clemmens, B., Calabro, F. J., Parr, A. C., et al. (2023). A canonical trajectory of executive function maturation from adolescence to adulthood. Nature Communications.

Steinberg, L., Icenogle, G., Shulman, E. P., et al. (2018). Around the world, adolescence is a time of heightened sensation seeking and immature self-regulation. Developmental Science.

Cohen, A. O., Breiner, K., Steinberg, L., et al. (2016). When is an adolescent an adult? Assessing cognitive control in emotional and nonemotional contexts. Psychological Science.

Pfeifer, J. H., & Allen, N. B. (2012). Arrested development? Reconsidering dual-systems models of brain function in adolescence and disorders. Trends in Cognitive Sciences.

Duell, N., Lansford, J. E., Rothenberg, W. A., et al. (2025). Positive risk taking across the world. Journal of Research on Adolescence.

Want to explore these ideas further? Learn more about my work on leadership, identity and conviction at www.theglennllopis.com.