(written from a Production point of view)
Robert Walker, Jr. (15 April 1940 – 5 December 2019; age 79), often credited simply as Robert Walker, was the actor who played Charles Evans in the Star Trek: The Original Series first season episode "Charlie X". Despite playing the seventeen-year-old Charles Evans, Walker was actually twenty-six at the time.
Walker was approached in 1997 for a role in Star Trek: Deep Space Nine's sixth season, but he was "not interested in renewing his acting career" according to Ira Steven Behr. Behr remembered: "When I lived in Malibu, Robert Walker, Jr. ran a store there and I used to see him on occasion. I used to think 'Oh my God, It's Charlie X'." (AOL chat, 1997)
Walker was the son of actor Robert Walker and Academy Award-winning actress Jennifer Jones. His younger brother Michael, who died on 23 December 2007, was also an actor. 
Walker made his film debut opposite Kirk Douglas and future Star Trek: The Next Generation 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 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).
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, Murder, She Wrote (starring William Windom) and Dallas (with Susan Howard, John Beck, and Morgan Woodward). 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.