Final answer:
The human brain completes its development by the early twenties, around 25 years old. The frontal lobes fully develop in this period, marking the end of significant cognitive and functional brain changes that began in infancy and continued through adolescence.
Step-by-step explanation:
The human brain completes its development by the early twenties, specifically by the age of 25. This is a period that surpasses infancy, childhood, and adolescence. During adolescence, while many cognitive abilities are at adult levels, the brain continues to develop in terms of judgment and insight well into the early twenties. The frontal lobes become fully developed around this time, which is critical for complex processes such as decision-making, personality expression, and moderating social behavior.
Childhood is biologically defined as the stage after infancy and before adolescence. However, it is during the transition from adolescence to adulthood that the brain undergoes significant structural changes, becoming more complex and efficient. Cognitive development that begins in infancy continues throughout childhood and peaks during adolescence, with the brain achieving approximately 90 percent of its adult size by the age of six. The process of synaptic pruning and increasing myelination further fine-tunes the brain during adolescence, leading to a period of rapid cognitive development.
In summary, while physical growth of the brain is mostly complete by early childhood, the developmental journey continues well into the early twenties with crucial cognitive and functional enhancements solidifying during this stage. This extended period of development allows for greater capacity in terms of insight and judgment, integral to the transition into effective adulthood.