Lost in Space - Season 3 - Eps 19: The Promised Planet
The Robot suddenly announces that the Jupiter II is approaching Alpha Centauri. Everyone is suprised, and the ship lands on a planet, where they are greeted by a group of people who appear to be teenagers from Earth. It turns out that they are aliens who live like Earth-style hippies, and they are incapable of growing old. They want to age normally, and to do this they want to get the older members of the Robinson party to leave in the Jupiter II while the younger members can stay behind and be used in an experiment that will make it possible for them to do so. The aliens trick the older members of the Robinson party into leaving, but Will pulls a fast one on the aliens and the Jupiter II eventually returns to the planet, and Penny, Will, and Dr. Smith, who somehow passed himself off as a young person, are rescued.
About Lost in Space

Title: Lost in Space
First Air Date: 1965-09-15
Last Air Date: 1968-03-06
Status: Ended
Rating: 7.1/10 (from 115 votes)
Language: EN
Seasons: 4
Total Episodes: 83
Network: CBS
Genres: Sci-Fi & Fantasy, Comedy
Production Companies: Irwin Allen Productions, 20th Century Fox Television
Synopsis
The space family Robinson is sent on a five-year mission to find a new planet to colonise. The voyage is sabotaged time and again by an inept stowaway, Dr. Zachary Smith. The family's spaceship, Jupiter II, also carries a friendly robot who endures an endless stream of abuse from Dr. Smith, but is a trusted companion of young Will Robinson
Cast

Guy Williams
Dr. John Robinson

June Lockhart
Dr. Maureen Robinson

Mark Goddard
Major Don West

Marta Kristen
Judy Robinson

Angela Cartwright
Penny Robinson

Bill Mumy
Will Robinson

Jonathan Harris
Dr. Zachary Smith

Dick Tufeld
Robot B-9

Bob May
Robot B-9