Final answer:
The brain shrinks by about 10 percent over a lifetime, with significant changes in complexity and efficiency occurring during adolescence and ongoing adaptability throughout life.
Step-by-step explanation:
Over a lifetime, the brain shrinks by about 10 percent. This change in brain size does not occur uniformly over a person's life but tends to happen as part of the natural aging process. Studies have shown that as we age, there can be a reduction in the volume of the brain. Although the brain reaches about 90 percent of its adult size by the age of six, changes continue throughout life. During adolescence, the brain undergoes significant changes in complexity, including increased folding in the cerebral cortex and synaptic pruning, which ultimately leads to more efficient brain function.
It's important to note that the brain also displays remarkable adaptability, known as neuroplasticity, throughout a person's life. For instance, new neurons can form (a process called neurogenesis), and neural pathways can continue to develop and refine, which is why lifelong learning and mental exercises can contribute to brain health even as we age.