When a human is born, the human brain is really incomplete. It has all of its neurons but very few where you have new neurons being born. Largely what's going to change is the wiring. And so the brains are waiting for a set of wiring instructions from the world. Those wired instructions partially come from the physical world. The extent to which you make eye contact with the baby will wire that infant's brain and this continues more or less for about 25 years. Generally speaking it takes 25 years to fully bake an adult brain, human brain.