four year olds
Like the frequent colds they acquire from their suspect hygiene practices, giggles are also contagious for four year olds. They don't understand war, taxes, poverty, genocide, politics or depression; they understand play. Play keeps them is a constant state of learning and therefore in a constant state of joy.
Tests, whichever age they are undertaken, are not simply a pressure vault where children are expected to perform, but can over time improve the long-term chances of children, and hopefully, their overall quality of life.