From Heroes Wiki
William Charlton is the actor who portrays a young Arnold.
William has appeared in the television shows Days of Our Lives, The Shield, General Hospital, Medium, Close to Home, and Judging Amy. He has appeared in the films Changeling, Flags of Our Fathers, and Boxed.
He did voicework on the video game L.A. Noire with Greg Grunberg, David H. Lawrence XVII, Kate Connor, Jared Ward, Benjamin Byron Davis, Ron Perkins, John Thaddeus, Wiley Pickett, Michael Silver, John Prosky, Ned Vaughn, Andrew Connolly, Dusty Sorg, and Ryan Smith.