Between 5 and 10 percent of preschoolers have vision problems, according to the Agency for Healthcare Research and Quality, part of the federal Health Department that recommends kids have their vision screened before age 5. Without treatment, conditions like lazy eye can affect a child's ability to learn, the agency says. So can run-of-the-mill nearsightedness or farsightedness that prevents a child from seeing the classroom chalkboard, says Gwen Glew, M.D., a developmental pediatrician at the University of Washington. "Kids can be cognitively normal but be failing at school because they can't see well," she says. Experts recommend school-age kids get their visual function tested anually by their pediatrician or family physician. But if you suspect your child is suffering from decreased vision, or if there are hereditary factors that might predispose your child to eye disease, make an appointment with an opthamologist.