A child's brain is not fully formed when they're newly born. In order to develop, the brain cells need to be activated to form connections, pathways and networks that give children the powers of vision, language, smell, muscle control and reasoning.
A child's brain development includes periods during which the brain is particularly efficient at learning. However these periods are only open for a defined time and neuroscientists have found that once a period closes, the brain structure becomes more difficult to change. If children miss out on experiences within these periods, they may not develop the richness or complexity of skills that they could be capable of.