I don't mean sidekicks, and not even started as sidekicks then became superheroes, I mean those who started as superheroes in their teens and I shall get non newer than those who I know appeared in the sixties.
1. Star Spangled Kid/Sylvester Pemberton (DC Comics): The original
Star-Spangled Kid was
Sylvester Pemberton, a
Golden Age character. He became the Star-Spangled Kid in order to battle
Nazi spies and
fifth columnists during
World War II. He was unique in that he was a kid superhero who operated with an adult sidekick,
Stripesy a.k.a. Pat Dugan (he sort of looks like Sandman/FlintMarko/William Baker/Sylvester Mann). Both he and Dugan were superb acrobats along with having sufficient training in hand-to-hand combat. They devised a series of acrobatic maneuvers that allowed them to build upon one another's strengths, the Kid's agility and Dugan's strength. They also built the Star Rocket Racer, a bubble-topped limousine with the functions of a rocket and helicopter.
This image is for Sylvester after growing-up and becoming the superhero Starman.
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2. Captain America/Steve Rogers: According to "Ed Brubaker" in "Captain America 601". He wanted to become a soldier but wasn't fit, then he underwent the transformation by the superhero serum when he was eighteen years old. Eight-Teen
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3. Johnny Storm/The Human Torch: Started as a member of a team of grown ups, not just a simple sidekick (Stan Lee sure loves to have his heroes start in a young age, at least not all of them started like that) but a very important and efficient team member. He even had his solo adventures back then.
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4. Spider-Man: This one is unique for being the first teenage solo superhero, not a sidekick, not a team player and didn't look for help in his fights, even when he got help offered to him he often rejects it because his fights with super villains are personal.
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5. Original X-Men by Lee & Kirby: X-Men started as five teenagers in an academy being run by "Professor Charles Xavier". Leader comes first so: Cyclopse, Jean Grey(the infamous phoenix, she will soon be resurrected be sure of that, or maybe any time within the next ten years if she isn't right now) Beast, Angel & Iceman