Some brain regions shrink after a certain age, but some never stop growing
The question of when the brain ‘stops’ growing has been one of conflicting nature for a long time. Before the 1970s, research suggested that you were born with all the neurons you will ever have in your life. However, there has been growing evidence to dispute that.
Essentially, research shows that the volume of the brain increases until one’s 20s/30s and starts declining after their 40s by approximately 5% of gray matter volume per decade, and much faster after age 70.
In order to further understand how much specific brain regions’ volumes change with age, I have first attached an image depicting the structural location of each brain region below for your reference.
Primarily, one’s prefrontal cortex (regulates planning and decision-making), and temporal lobe (involved in processing language) is shown to decrease significantly after age 70. The occipital cortex, involved in vision, decreased the least in size as compared to other brain regions.
MRI studies have found that corpus callosum (that divides the two brain hemispheres) grows in size up to one’s late 20s, and declines with old age.
The thalamus, amygdala, putamen, nucleus accumbens sizes declined with age(largely after 40s/50s) - these regions regulate information processing, emotion, movement, and the brain’s ‘reward circuit’.
Grey matter volume in the cerebral cortex (i.e, the outer layer of the cerebral hemisphere, largely responsible for higher-order thinking) found to decline by approximately 10% as one aged from their 30s to their 70s, and by around 12% by the time one reached 80. White matter volume, however, increased until age 50 and then declined at a comparable rate.
Hippocampal (involved in memory formation & spatial ability) volume actually remained relatively stable throughout life, and this perhaps links to its ability to regenerate neurons. It has been largely found that new neurons develop in the hippocampus forever, and this increase in neurons has shown to have antidepressant effects. In fact, there is an estimated 700 hippocampal neurons made daily in the adult hippocampus, which dips slightly at old age. However, there has also been some opposing evidence on this matter, where no new neurons in the adult hippocampus were found.
In all, Total Brain Volume declined strongly after age 60.
In summary, the brain stops ‘growing in size’ at approximately one’s 30s-40s, after which it declines moderately, and then more steeply after one reaches their 60s/70s, and even more after age 80.