Kyung Hee Kim, The Creativity Crisis: The Decrease in Creative Thinking Scores on the Torrance Tests of Creative Thinking, November 9, 2011:
Children’s ability to produce ideas (Fluency) increased up to third grade and remained static between fourth and fifth grades, and then continuously decreased, which might indicate children become alert to issues like accuracy and appropriateness of their responses when they generate ideas. During middle childhood, children are more concerned about representational accuracy rather than aesthetic appeal itself (Rosenblatt & Winner, 1988). Increases in preference for appropriate ideas and in evaluative thinking are related to decreases in divergent thinking (Charles & Runco, 2001; Runco, 2003).
Children’s ability to think in a detailed and reflective manner as well as their motivation to be creative (Elaboration) increased steadily until high school, when it is static, and then decreases in adulthood. This may indicate children are increasingly willing to elaborate and are rewarded for it through their school years, and the rewards for elaboration decreased after graduation, and elaboration may distract from post–high school pursuits. This is true only for the general adult population, and not for all adults. For eminent creative adults, elaboration increases with age. Simonton (1983) suggested eminent creative adults generate a lot of ideas in their earlier careers, but later they focus on elaboration of their ideas, and creative productivity increases as elaboration increases. Thus, creative productivity for eminent creators does not decrease with age but increases with age and level of elaboration
See also, Joel Stein, Is human creativity fading away?