Final answer:
The brain reaches 95% of its development by the age of 25, especially the frontal lobes, which are crucial for higher-level cognitive functions. Adolescence brings significant brain changes, but the brain's structural maturation continues into the early 20s.
Step-by-step explanation:
When discussing brain development, it is important to understand that the brain continues to grow and change well into the early 20s. The frontal lobes, in particular, which are associated with planning, reasoning, memory, and impulse control, become fully developed by the age of 25. Thus, in response to the student's question: At what age has the brain reached 95% of its development? The answer is d) 25 years old.
Adolescence is a time of rapid cognitive development, with significant changes happening in the brain structure. For instance, the number of folds in the cerebral cortex increases and synaptic pruning occurs, making the brain more efficient. By six years of age, the brain has about 90 percent of its adult size, but it continues to develop in complexity through the teenage years and into the early 20s, specifically with increases in myelination and changes in the ratio of gray matter to white matter.
Thus, while basic cognitive skills such as attention and memory are comparable to adults by around 15 years old, the full maturity of cognitive functions such as decision making and impulse control aren't fully refined until around the age of 25, which corresponds with the complete development of the frontal lobes.