Before there was the Teen Titans, there was The Legion of Super-Heroes. The team was founded by three teenagers from different planets: Cosmic Boy, Lighting Boy (later renamed Lighting Lad), and Saturn Girl. They were created by Otto Binder (co-creator of the original Captain Marvel) and Al Plastino, and made their first appearance in Adventure Comics #247. In the story, the Legion travels from their time in the 30th Century to Smallville in the early years of the 20th Century. There, the three founders met Superboy, the younger version of Superman, and tell him that he is their inspiration. They also decide to induct him into their ranks. The team was originally only going to appear in one story, but the comic did so well that the team returned two years later in Adventure Comics #259. By the 1960’s, the Legion would recruit the following members: Chameleon Boy, Invisible Kid, Colossal Boy, Star Boy, Brainiac 5, Triplicate Girl (who would later become Duo Damsel after one of her copies died), Shrinking Violet, Sun Boy, Bouncing Boy, Phantom Girl, and Ultra Boy. They would become a regular feature in Adventure Comics #300.
The Legion had two rules for admittance. The first was that you could only be a teenager, and the second was that you had to have at least one super power. When a member matured into adulthood, that member had to leave the team, and have their memories erased so that they wouldn’t reveal the Legion’s secrets.
Not every superhero had what it takes to make the Legion. In Adventure Comics #306, Polar Boy, Night Girl, Chlorophyll Kid, and Fire Lad tried to join, but were refused because their powers were too weak or uncontrollable. (Night Girl’s powers only work at night, for instance)
In 1964, a young boy named Jim Shooter sent scripts to DC, impressing them so much that he became a new writer. Beginning with Adventure Comics #346, Shooter became a new writer for DC, creating new members such as Karate Kid, Ferro Lad, Nemesis Kid, and Ferro Lad, as well as villains like The Fatal Five and Parasite.