Most people remember Robbie Rist from his his work on The Brady Bunch, in the early 70s. Rist, a miniature John Denver look-alike, played Cousin Oliver during the Bunch's jump the shark period.
My favorite Robbie Rist role, though, is his character Little John Martin in the 1976 Saturday live action kids show, Big John, Little John. In the show's pilot, middle-aged teacher Big John Martin discovers the legendary fountain of youth, drinks from it and magically--and unexpectedly--transforms into a child (Rist). Martin changed from adult to child and back several times each episode, but was never able to control the power, resultiung in madcap adventures. I loved the show, even though it was only on for one season.