Final answer:
The question addresses brain development through different stages, with the forebrain, midbrain, and hindbrain growing at variable rates and the forebrain becoming significantly larger by adulthood. Synaptic pruning and increased myelination contribute to the brain's complexity and efficiency during adolescence, concluding with the full development of the frontal lobes by 25 years old.
Step-by-step explanation:
The question discusses the maturation of the brain and the different timetables of development for various regions. Comparative embryologic development reveals that the forebrain, midbrain, and hindbrain, which are initially similar in size during the early embryonic stage, develop at different rates. By adulthood, the forebrain becomes significantly larger relative to the midbrain and hindbrain.
From a physiological perspective, the cerebral cortex in the brain experiences increased folding, leading to greater complexity. The process of synaptic pruning eliminates unused pathways while simultaneously, myelination increases to enhance the brain's efficiency and functionality, indicating significant cognitive development during adolescence.
Regarding the frontal lobes, full development is typically reached by 25 years old. In fetal development, critical periods are specific times when different parts or organs develop, which is crucial for understanding overall brain development.