Complex sentence: Kelly Ryan got an early start in her fitness career when at age eight she began to train in gymnastics, which would later serve as a foundation for other athletic activities she became involved in while in high school, such as cheerleading and basketball.
Split into simpler sentences: Kelly Ryan got an early start in her fitness career when, at age eight, she began to train in gymnastics. This would later serve as a foundation for other athletic activities she would become involved with in high school, such as cheerleading and basketball.