Ben Tennyson was abducted by aliens when he was 10 years old after accidentally received the Omnitrix, but then got rescued by Tetrax. He also brought Rook Blonko with his side. Both Ben and Rook are orphans, where Tetrax trained them to become the Galactic Guardians. Then after reaching his teen years, he received the Ultimatrix. At 18 years old, Ben and the Galactic Guardians decides to come to Earth, where they met the Justice League. (NOTE: this animation will be based from DCAU, but I've rebooted the story, plus I've combined it with Ben 10's Universe, the DC Universe, and TMNT's Universe)

dcau timeline

Ben 10/Justice League: The Animated Series