When I researched this on Wikipedia I found that I’m not the only person to make that link.
Robert A. Heinlein was one of the three people seen as pillars of the Golden Age of science fiction (that’s another post). He wrote many books and short stories, but a period of his novels are considered as the start of YA, although he didn’t consider them as such.
The books that I’m referring to are his juveniles.
Between 1947 and 1958e had 12 such novels published by Scribner, with another (Starship Troopers) published by Putnam instead (Scribner rejected it) and another novel (Podcayne of Mars) listed as a juvenile, though he didn’t consider it one.
Heinlein didn’t consider these books as juveniles, at least not by nature. They were written for younger readers, but Heinlein had great respect for these younger readers, so he tried to write more challenging fare for them. In fact, this got him into hot water with his editors at Scribner, and often, after he brought guns into his novels, starting with Red Planet.
He also wrote 2 short stories in Boy Scout magazine, Boy’s Life. He created them after his tours in WW II, trying to diversify his writing from only pulp SF magazines. These stories were serialized.
Not only focussed on boys, he took a challenge to write for girls too, which led to 3 Maureen “Puddin'” stories in Calling All Girls magazine. He liked the character so much that he lowered her weight and relocated her to Mars for Podkayne of Mars.
I have to re-read this book. The original ending was hated by fans, so Heinlein rewrote it, then had it published with both endings. I don’t remember what they were. It’s been more than 3 decades since I read it.
Pundits call it a juvenile, but Heinlein himself did not. His involvement with Scribner and the juveniles line ended when they rejected Starship Troopers. As an aside, I don’t see a novel about interstellar wars as a book for young people, it was just not a great book to me.
What made the juveniles a step toward YA: youths are the protagonists of the stories. Not bad, considering they were written nearly 80 years ago.