TWO & THREE YEAR OLDS
When children are developing verbal, motor and social skills
THREE TO SIX YEARS OLD
The "Absorbent Mind" stage, when children are learning very rapidly--mostly about concrete ideas and concepts.
SIX TO NINE YEARS OLD
When children begin to grasp abstract concepts, and when they really start to think and find answers by themselves.
NINE TO TWELVE YEARS OLD
When children have the capability (and the opportunity) to research subjects themselves, and plan their own projects and activities.