It's been thought that the neurons were already created before birth, but research indicates that the brain keeps growing until mid to late childhood, decreases through the early 20s, stabilizes for quite a while, and then begins to decrease further from age 40 onwards.

Brain Growth

We might have this idea that our brains change the same way but scientists have observed that there is a considerable variation between individuals in terms of when and what to extent aspects of the brain change.
Individual differences in brain development can be related to psychological differences.

Everyone's Brain Changes Differently

Brain development happens at different rates for different parts of the brain but here are the major trends in the construction that takes place:
<ul>
<li>In the womb: folding of the brain's outer layer</li>
<li>Newborn - childhood: the brain grows gradually and continues to develop white matter pathways and maturity of movement and sensation</li>
<li>Adolescence: gray matter thins out; synapses scale back, and susceptibility to outside influence heightens</li>
<li>Early adulthood: the prefrontal cortex is still developing.</li>
</ul>

Stages of Early Brain Development

<ul>
<li>Neurogenesis is the process by which new neurons are created. The bulk of this process happens during the prenatal period of brain development.</li>
<li>Synaptogenesis is the process of forming connections or synapses.</li>
<li>Synaptic pruning is the process where synapses that aren't used grow weaker and disappear.</li>
<li>Myelination is a key process in neuronal development that begins before birth</li>
</ul>

Neurogenesis and Neuronal Development

<ol>
<li>Genes - are a major key factor in brain development due to their contribution to normal individual variation in brain development, and relatively rare genetic alterations.</li>
<li>Environmental factors - Because having a safe and nurturing environment facilitates proper growth.</li>
</ol>

Major Factors That Influence Brain Development

<ul>
<li>Also known as neural plasticity</li>
<li>This is the capacity of the brain to change in response to experience and the environment.</li>
<li>It changes the brain from updating the neuronal networks to the formation of new memories to larger structural adaptations. </li>
<li>The brain has a great amount of plasticity and it remains an essential quality of the brain throughout adulthood.</li>
</ul>

Neuroplasticity

Before a person is even born, the brain has undergone intense expansion and growth in its complexity. This dramatic development continues into the first years of childhood. After an early boom period, a process of refinement and reduction sets in: childrens' and adolescents' brains respond to their life experiences and the world around them, and the most-used connections between brain cells are strengthened while others are pruned.

Brain Growth

How the Brain Develops | Psychology Today

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