(written from a Production point of view)
Robert Walker, Jr. (born 15 April 1940; age 75), often credited simply as Robert Walker, is the actor who played Charles Evans in the first season episode of Star Trek: The Original Series entitled "Charlie X". Despite playing the seventeen-year-old Charles Evans, Walker was actually twenty-six at the time.
Walker was required on the set all shooting days during the production of the episode, filming his scenes on Monday 11 July 1966, Tuesday 12 July 1966, Wednesday 13 July 1966, Thursday 14 July 1966, Friday 15 July 1966, Monday 18 July 1966 and Tuesday 19 July 1966, all at Desilu Stage 9.
Walker is the son of actor Robert Walker and Academy Award-winning actress Jennifer Jones. His brother, Michael, is also an actor. Michael Walker died on 27 December 2007. 
Walker made his film debut opposite Kirk Douglas and future TNG guest actor Nehemiah Persoff in The Hook, which opened in 1963. Walker went on to gain some fame for his supporting role in the 1963 film The Ceremony, for which he received a Golden Globe award as Most Promising Male Newcomer. He followed this with the starring role in 1964's Ensign Pulver, playing the titular character.
In 1967, Walker co-starred again with Kirk Douglas and future TOS guest actress Valora Noland (Daras in Patterns of Force) in the John Wayne western classic, The War Wagon. In 1968, Walker co-starred with fellow TOS guest star Celeste Yarnall in the thriller The Face of Eve. He followed this with a role as a communal hippie in the 1969 Academy Award-nominated classic Easy Rider. TOS/TNG guest star Michael Pataki and TOS guest star Sabrina Scharf also appeared in this film.
His other film credits include 1969's Young Billy Young (with John Anderson and Paul Fix), 1972's Beware! The Blob (with Richard Webb and Gerrit Graham), and 1984's Hambone and Hillie (with Wil Wheaton). He also had a role in the acclaimed 1980 film The Long Good Friday.
Besides his role on Star Trek, his other television appearances include the made-for-TV movies Columbo: Mind Over Mayhem (1974, with Lou Wagner, Arthur Batanides and Charles Macaulay), Making of a Male Model (1983, with Joan Collins, Patricia Smith and Robert DoQui), and Fatal Charm (1990, with Andrew Robinson), as well as episodes of Twelve O'Clock High (starring Frank Overton and Robert Lansing), Bonanza, The F.B.I., The Six Million Dollar Man, and Murder, She Wrote (starring William Windom). He also guest-starred as Billy the Kid in an episode of The Time Tunnel, starring Whit Bissell, James Darren, and Lee Meriwether (the episode also guest starred John Crawford), and had a role in the 1980 television mini-series Bealah Land, as did Jonathan Frakes and Michael Sarrazin.