Complex sentence: At SummerSlam, Lesnar defeated The Rock to become the WWE Undisputed Champion and youngest WWE Champion in history at age 25 (a record previously held by The Rock and stands to date).
Split into simpler sentences: At SummerSlam, Lesnar defeated The Rock to become the Undisputed WWE Champion. With his victory, Lesnar became the youngest WWE Champion in history at age 25 (a record that was previously held by The Rock and stands to date).