Osgood-Schlatter disease is a condition that causes pain and swelling just below the knee, where the patellar tendon attaches to the tibia (shinbone). It typically affects active adolescents who are going through growth spurts during puberty. The condition is most common in children aged 10 to 15 years and is often seen in those who participate in sports that involve running, jumping, or quick directional changes, such as soccer, basketball, or gymnastics.