Use it or lose it. At age 3, the number of synaptic connections that make up the brain’s neural network is at its peak. In fact, a 3-year-old’s brain is twice as active as an adult’s brain. At this point, brain development doesn’t involve growing new neural connections as much as strengthening existing connections through repeated use and a natural pruning process that causes underutilized pathways to wither away.
Zero to Three on brain development. Available at: http://www.zerotothree.org/child-development/braindevelopment/