Billy Mumy came to prominence in the 1960s as a child actor, most notably as Will Robinson in the CBS sci-fi television series Lost in Space. He later appeared as a lonely teenager, Sterling North, in the 1969 Disney film, Rascal. He was cast as Teft in the 1971 film Bless the Beasts and Children. In the 1990s, he had the role of Lennier in the syndicated sci-fi series Babylon 5, and he also served as narrator of the Emmy Award winning series, Biography
The only person on this list that actually pulled off being a teenager!
I did that with Mark Shepard who is in Doctor Who and Supernatural. I saw him on Supernatural and then saw him on Doctor Who I yelled " That's the King of Hell!" Everyone looked like I was insane.