One day last year when her daughter, Ainsley, was 9, Tracee Sioux pulled her out of her elementary school in Fort Collins, Colo., and drove her an hour south, to Longmont, in hopes of finding a satisfying reason that Ainsley began growing pubic hair at age 6. Ainsley was the tallest child in her third-grade class. She had a thick, enviable blond-streaked ponytail and big feet, like a puppy’s. The curves of her Levi’s matched her mother’s.