Are You Really a Different Person at 25? The Truth Behind the “Fully Developed Brain”

If you’ve spent any time on TikTok, Instagram or YouTube lately, you’ve probably come across videos explaining why, apparently, turning 25 changes everything about your brain. You may have heard the phrase, “Your brain isn’t fully developed until you’re 25.” It is one of the most common neuroscience claims online, often used to explain impulsivity, poor decision-making, or why young adults appear to become more mature once they reach 25. 

Examples of content creation on social media platforms relating to the frontal lobe completing its development at the age of 25.

Although the claim does have some basis in genuine neuroscience research, it dramatically simplifies what actually happens during brain development. There is no single point at which the human brain suddenly becomes “fully developed”, nor is there evidence for a universal biological switch that occurs on someone’s 25th birthday. Instead, brain maturation is a prolonged and dynamic process involving changes in grey matter, white matter, synaptic connectivity, myelination, functional networks and gene expression. These changes do not follow an identical timeline for everyone, meaning that the rate and timing of brain development and maturation can vary considerably between individuals.

Where did the age of 25 come from?

The idea of 25 is partly linked to early MRI research showing that brain development continues throughout adolescence and that the prefrontal cortex is among the later-developing regions. However, the often-repeated age of 25 was not identified by these studies as a biological finishing point. Giedd et al. (1999), for example, followed brain development from childhood into early adulthood but did not conclude that the brain becomes fully developed at 25. The number appears to have become attached to this research as scientists continued to investigate development into the twenties, before being simplified in popular discussions into the claim that the “frontal lobe finishes developing at 25”. Subsequent research has instead shown that different brain regions and biological measures follow different developmental trajectories, making a single finishing age difficult to define (see Figures 1 and 2).

Figure 1. Regional trajectories of cortical grey matter volume across childhood and adolescence. The graphs show how grey matter volume changes with age in different areas of the cortex. Adapted from Gogtay et al. (2004).

Figure 2. Changes in cortical grey matter from childhood to early adulthood. The maps show changes in grey matter across the cortical surface between approximately 5 and 20 years of age. Adapted from Gogtay et al. (2004).

Together, these figures show that brain development does not follow one uniform timeline. Figure 1 shows that different cortical regions follow different patterns of grey matter change, while Figure 2 allows us to visualise how these changes occur across the cortex over time. Rather than the whole brain reaching maturity at once, different regions change at different rates and at different ages. This helps explain why defining a single age, such as 25, at which the entire brain becomes “fully developed” oversimplifies a much more complex process.

What is actually happening inside the developing brain?

Adolescence and early adulthood stimulates extensive changes in the brain. One of these is synaptic pruning, where some connections between neurons are removed while others are strengthened, helping brain networks to become more efficient. Myelination also increases during this period; this is when cells called oligodendrocytes produce myelin, a fatty substance that surrounds nerve fibres to help signals travel more quickly and efficiently through the brain.

Grey matter changes also vary across different brain regions: some areas become thinner as we develop, but this does not necessarily mean they are becoming less developed! Instead, these changes can reflect the brain refining and reorganising its connections. In fact, this type of refinement happens extensively much earlier in development too. During infancy and early childhood, the brain initially produces an abundance of synaptic connections between neurons, creating more connections than will eventually be needed. As a child develops and interacts with their environment, frequently used connections can be strengthened while others are weakened or eliminated, a process known as synaptic pruning (Huttenlocher, 1990). This allows neural circuits to become increasingly specialised and efficient over time. Importantly, this shows that losing or reorganising connections can be a normal and important part of brain development rather than evidence that a brain is becoming “less developed”. Therefore, changes in grey matter are not simply a sign that the brain is gaining or losing tissue, and a single measurement from an MRI scan cannot tell us whether a brain is simply “developed” or “undeveloped”.

The prefrontal cortex is a particularly important brain structure due to its involvement in functions such as planning, working memory and controlling impulses. The dorsolateral prefrontal cortex is one of the later-maturing areas of the brain, with Giedd (2004) describing it as “among the latest brain regions to mature”. However, this does not mean it is “unfinished” until the age of 25. Instead, its development involves gradual changes in its structure, connections and organisation, making brain development a process of continual refinement rather than a sudden endpoint.

How does this correlate with behaviour?

Changes in brain networks during adolescence can influence how we respond to rewards, emotions and social situations. As the brain develops, connections between regions are continually refined and strengthened, while processes such as myelination help signals travel more efficiently through neural networks. However, these changes do not occur at the same rate across all systems. Blakemore and Robbins (2012) describe “the relatively slow, linear development of impulse control and response inhibition”, while the brain’s response to rewards follows a different pattern of development.

One important network is the frontostriatal system, which connects the prefrontal cortex with the striatum - a region involved in processing rewards and helping guide behaviour. Other areas, including the limbic system, are involved in emotions and motivation. As these systems develop and interact, they may contribute to greater sensitivity to rewards or social influences in certain situations, such as being more likely to take a risk when surrounded by friends than when making the same decision alone.

However, this does not imply that everyone below the age of 25 has poor impulse control, or cannot make rational decisions. Behaviour is influenced by much more than brain development, including experience, learning, the environment and individual differences. Someone can show strong self-control in one situation and act impulsively in another; neuroscience cannot neatly divide people into those with “immature brains” below 25 and those with “mature brains” above 25.

Does brain development actually stop at 25?

If brain development continues into the twenties, it is reasonable to question whether 25 is, at least, a useful approximation for the cessation of brain maturation. The problem is that different measures of brain maturation produce different timelines; grey matter, white matter, cortical thickness, functional connectivity and molecular changes do not all reach a common endpoint and, even within the prefrontal cortex, different cell populations and neural circuits can mature at different rates. More recent single-cell research has revealed changes in gene regulation across development, showing “continuous reconfiguration into adulthood”. This highlights that maturation is not simply a structural process visible on an MRI scan, but also involves changes in how genes are regulated and how different brain cells function.

Furthermore, there is no singular point at which the adult brain becomes biologically static. Neuroplasticity allows neural circuits to change in response to learning, experience and environmental demands (see our article on neuroplasticity here). Research into adult learning has shown that regional grey matter volume and cortical thickness can be experience-dependent, demonstrating that the brain remains capable of structural and functional adaptation beyond adolescence and young adulthood. This further undermines the idea that there is a single age at which the brain becomes “fully developed”.

So, does anything unique happen when you turn 25?

Nothing dramatic happens to the brain at 25. You do not suddenly develop a “finished” prefrontal cortex or acquire perfect impulse control. Instead, your brain has undergone years of gradual structural, functional and molecular changes since childhood, with some processes continuing throughout adulthood. The age of 25 has therefore become a convenient shorthand for the more complex finding that some aspects of brain maturation extend into the twenties, although the original research never established a universal biological deadline. The brain is not an unfinished organ waiting to be completed, but a dynamic biological system that is continuously remodelled and remains plastic throughout adulthood. So, while your brain at 25 may be different from your brain at 15, there is no neuroscientific finish line waiting at the end of your 24th year!  The science is much more interesting than that.

References

• Arain, M., Haque, M., Johal, L., Mathur, P., Nel, W., Rais, A., Sandhu, R. and Sharma, S. (2013). Maturation of the adolescent brain. Neuropsychiatric Disease and Treatment, 9, pp.449–461. doi:10.2147/NDT.S39776.

• Blakemore, S.-J. and Choudhury, S. (2006). Development of the adolescent brain: implications for executive function and social cognition. Journal of Child Psychology and Psychiatry, 47(3–4), pp.296–312. doi:10.1111/j.1469-7610.2006.01611.x.

• Blakemore, S.-J. and Robbins, T.W. (2012). Decision-making in the adolescent brain. Nature Neuroscience, 15(9), pp.1184–1191. doi:10.1038/nn.3177.

• Caballero, A., Granberg, R. and Tseng, K.Y. (2016). Mechanisms contributing to prefrontal cortex maturation during adolescence. Neuroscience & Biobehavioral Reviews, 70, pp.4–12. doi:10.1016/j.neubiorev.2016.05.013.

• Casey, B.J., Jones, R.M. and Hare, T.A. (2008). The adolescent brain. Annals of the New York Academy of Sciences, 1124(1), pp.111–126. doi:10.1196/annals.1440.010.

• Giedd, J.N. (2004). Structural magnetic resonance imaging of the adolescent brain. Annals of the New York Academy of Sciences, 1021(1), pp.77–85. doi:10.1196/annals.1308.009.

• Giedd, J.N., Blumenthal, J., Jeffries, N.O., Castellanos, F.X., Liu, H., Zijdenbos, A., Paus, T., Evans, A.C. and Rapoport, J.L. (1999). Brain development during childhood and adolescence: a longitudinal MRI study. Nature Neuroscience, 2(10), pp.861–863. doi:10.1038/13158.

• Gogtay, N., Giedd, J.N., Lusk, L., Hayashi, K.M., Greenstein, D., Vaituzis, A.C., Nugent, T.F., Herman, D.H., Clasen, L.S., Toga, A.W., Rapoport, J.L. and Thompson, P.M. (2004). Dynamic mapping of human cortical development during childhood through early adulthood. Proceedings of the National Academy of Sciences, 101(21), pp.8174–8179. doi:10.1073/pnas.0402680101.

• Herring, C.A., Simmons, R.K., Freytag, S., Poppe, D., Moffet, J.J.D., Pflueger, J., Buckberry, S., Vargas-Landin, D.B., Clément, O., Goñi Echeverría, E., Sutton, G.J., Alvarez-Franco, A., Hou, R., Pflueger, C., McDonald, K., Polo, J.M., Forrest, A.R.R., Nowak, A.K., Voineagu, I., Martelotto, L. and Lister, R. (2022). Human prefrontal cortex gene regulatory dynamics from gestation to adulthood at single-cell resolution. Cell, 185(23), pp.4428–4447.e28. doi:10.1016/j.cell.2022.09.039.

• Johnson, S.B., Blum, R.W. and Giedd, J.N. (2009). Adolescent maturity and the brain: the promise and pitfalls of neuroscience research in adolescent health policy. Journal of Adolescent Health, 45(3), pp.216–221. doi:10.1016/j.jadohealth.2009.05.016.

• Lövdén, M., Wenger, E., Mårtensson, J., Lindenberger, U. and Bäckman, L. (2013). Structural brain plasticity in adult learning and development. Neuroscience & Biobehavioral Reviews, 37(9), pp.2296–2310. doi:10.1016/j.neubiorev.2013.02.014.

• Sowell, E.R., Thompson, P.M., Holmes, C.J., Jernigan, T.L. and Toga, A.W. (1999). In vivo evidence for post-adolescent brain maturation in frontal and striatal regions. Nature Neuroscience, 2(10), pp.859–861. doi:10.1038/13154.

• Huttenlocher, P.R. (1990). Morphometric study of human cerebral cortex development. Neuropsychologia, 28(6), pp.517–527. doi:10.1016/0028-3932(90)90031-I.

  
  

---

This article was written by Saajidah Mohamud and edited by Rebecca Pope, with graphics produced by Eve Cottenden. If you enjoyed this article, be the first to be notified about new posts by signing up to become a WiNUK member (top right of this page)! Interested in writing for WiNUK yourself? Contact us through the blog page and the editors will be in touch.