cornettes-racket, as Morgus says, the Captain Marvel movie serial is well worth checking out, but then, I'm a fan of movie serials and Cap is one of the best. Just remembered, it's available on youtube here:-
https://www.youtube.com/watch?v=bItmKww9aWwIf that tickles your fancy, we have links to a number of old serials, incl. The Phantom Empire, Undersea Kingdom, The Phantom, both Green Hornet serials and more:-
https://comicbookplus.com/?cid=2949Here's a BBC piece with Monkhouse talking about Radio Fun:-
http://www.bbc.co.uk/radio4/comedy/radiofun.shtmlThere aren't many boy into adult superheroes around but in British comics there were a few characters who changed into completely different superheroes, i.e. without doing the Superman/Batman thing of simply taking off their clothes revealing the super suit underneath. Captain Conquest, a little known superhero who appeared in Knockout Fun, whispered the word of power, "Kagaran", and became Captain Conquest. You can read a story here:-
https://comicbookplus.com/?dlid=39639The Phantom Viking in Lion did a sort of Thor thing, where donning an ancient viking helmet changed him into The Phantom Viking via the power of his ancestors.
Another Mick Anglo hero was Captain Miracle, virtually a copy of Marvelman. Young lad shouts El Karim and changes form and size.
Anglo also had Captain Universe. Yet another magic word, GALAP, short for stuff. But the human is a young adult here.
A complete rip-off of Captain Marvel was Mr. Apollo and that is a kid turning into a super powered adult.
As K1ngcat mentions, there is Electroman.
Perhaps the best known was Thunderbolt Jaxon. Classic young lad becoming adult superhero. We have some stories here:-
https://comicbookplus.com/?cid=2046By the way, Monkhouse was also a film star.