Complex sentence: He became a bull rider at rodeos while he was still a teenager, but at the age of 15 was stricken with polio; he took up the guitar and the mandolin as therapy to recover the strength and dexterity of his arms and hands.
Split into simpler sentences: He became a bull rider at rodeos while he was still a teenager, but at the age of 15, he was stricken with polio. He took up the guitar and the mandolin as physical therapy to recover the strength and dexterity of his arms and hands.