The older a child gets the more their experiences cement the synapses in their brains. As time goes by the brain prunes the synapses that go unused.
The myelination process isn’t completely finished until about age 25 so the brain undergoes a lot of change between age three and 25. Throughout this time new synapses are built and positive experiences continue to further the development of the brain.
Adults experience a slower rate of brain development due to the decreased speed of synapse formation. This isn’t to say that new formations do not occur, just that the way they are built changes over time.
The brain never stops changing throughout a human’s entire life. There is rapid development in the early years which peaks in adolescence and plateaus nearing adulthood. Even well into adulthood positive experiences such as traveling, learning a new hobby, and being part of a strong support system will continue to advance the brain’s development and growth.
Negative experiences in adulthood also have an effect on brain development. Adults who have witnessed major catastrophes such as terrorist attacks will recall their experiences and develop a memory and reaction because of this.
Their habits may change, their opinions may change, and their fears may change. This is because of new connections in the brain due to the experiences that this person has had. Likewise, someone who witnessed footage of a catastrophic event on social media will also make new connections but these will be mild in comparison. This may be a positive or tolerable stress experience that the person can learn and grow from, whereas the former example would be toxic due to its extreme nature.