Final answer:
The human brain has the most connections between neurons during early childhood, in the first few years of life, during the blooming phase. Synaptic pruning follows through childhood and adolescence, streamlining the brain's efficiency, with full frontal lobe development by 25 years old.
Step-by-step explanation:
The age at which a human brain has the most connections between its neurons is during early childhood, specifically during the first few years of life. This period of rapid neural growth is referred to as blooming, where the brain experiences a surge in the formation of neural connections. The blooming phase is then followed by synaptic pruning through childhood and adolescence, a process where the brain sheds unused neural pathways to become more efficient. The frontal lobes, pivotal for complex cognitive functions, become fully developed by 25 years old.