Legg-Calve-Perthes disease can affect children of nearly any age, but it's most common among boys ages 4 to 8. In fact, it's about five times more common in boys than in girls. When girls develop Legg-Calve-Perthes disease, it tends to be more severe. In addition, Legg-Calve-Perthes disease is most common in whites. The disease may be more likely in physically active children who are small for their age and in those who are exposed to secondhand smoke.