I saw DeafCatMeow's post referencing the claim that the brain completes it's development at 25. Although I am NOT a scientist (IANAS), I'd read previously that this was a misconception so I decided to do some digging.

The myth that brain maturation is completed at 25 is a pop-culture distortion of the process of adolescent brain development: Basically, adolescence is one of the periods of major synaptogenesis (the creation/rerouting of new connections in the brain), but it isn't accurate to say that there is a 'cutoff' point or a switch at which the brain becomes fully mature. Neural plasticity is highest in children, but is retained throughout your life, which is what allows you to learn or 'make room for' new information.

This article, Maturation of the Adolescent Brain, cites this study, Sexual and Reproductive Health of Persons Aged 10-24 years for their claim that 'It is well established that the brain undergoes a “rewiring” process that is not complete until approximately 25 years of age.' As far as I can tell, this is referencing that Sexual and Reproductive Health studies youth up until the age of 24– a threshold that as far as I can tell was chosen because data was available in 5-year increments for study (10-14, 15-19, 20-24).

A more substantive claim from that article is that the prefrontal cortex, which is involved in behavioural control, "matures independent of puberty and continues to evolve up until 24 years of age", and that this is to what the 25-years as a milestone of maturity refers to.

Kolk and Rakic state that "the constantly developing cognitive and executive capabilities occur parallel to the neurophysiological changes within the PFC and its connected areas and seem to reach a plateau in teenagers" at around 12 years of age in humans, but this doesn't necessarily refer to maturity, as they also note that "higher order cognitive functions, in which [the prefrontal cortex] plays a prominent role, such as language and intelligence, continue to develop into adulthood."

Somewhat supporting the the 25-years-old argument is a study by Petanjek et. al, who conclude that "overproduction and developmental remodeling, including substantial elimination of synaptic spines, continues beyond adolescence and throughout the third decade of life before stabilizing at the adult level".

Petanjek et. al also write that "The end of the critical period of synaptic spine elimination in the human cortex basically relies on the pioneering study of Huttenlocher and colleagues (2, 4). Thus, it usually is assumed tacitly that the period of synaptic overproduction in the human cerebral cortex is completed by the end of puberty (18), even though Huttenlocher's study contains only a single 19-y-old brain specimen in the age group between 15 and 32 y."

Based off of Petanjek et. al. (again, IANAS!), there are major changes in the brain that can continue up until your 20s, but there doesn't seem to be a basis for the conclusive statement that 25 is when the brain stops maturing.

sources: https://www.sciencefocus.com/comment/brain-myth-25-development https://www.iflscience.com/does-the-brain-really-mature-at-the-age-of-25-68979 https://slate.com/technology/2022/11/brain-development-25-year-old-mature-myth.html* https://www.dovepress.com/article/download/12651 https://pubmed.ncbi.nlm.nih.gov/19609250/ https://www.nature.com/articles/s41386-021-01137-9 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3156171/

*edited to add an article