http://www.newsweek.com/2010/07/10/the-creativity-crisis.html

“In middle childhood, kids sometimes create paracosms — fantasies of entire alternative worlds. Kids revisit their paracosms repeatedly, sometimes for months, and even create languages spoken there. This type of play peaks at age 9 or 10, and it’s a very strong sign of future creativity. A Michigan State University study of MacArthur “genius award” winners found a remarkably high rate of paracosm creation in their childhoods.”