Rocket Ship Galileo, published in 1947, was the first of the “Heinlein juveniles”, aimed at what publishers now call the “young adult” market. The above was from a contemporary letter regarding his views on his choice of the teenage protagonists in the novel.
This is a rip-roaring adventure which remains readable today: here is my review from my most recent re-reading in 2015. The enduring popularity of these novels owes a great deal to Heinlein’s never talking down to his audience: the books are “juvenile” only in avoiding racy topics, but thoroughly enjoyable by adults. As I wrote in my review,
The remarkable thing about this novel is how well it holds up, almost seventy years after publication. While Heinlein was writing for a young audience, he never condescended to them. The science and engineering were as accurate as was known at the time, and Heinlein manages to instill in his audience a basic knowledge of rocket propulsion, orbital mechanics, and automated guidance systems as the yarn progresses. Other than three characters being young people, there is nothing about this story which makes it “juvenile” fiction: there is a hard edge of adult morality and the value of courage which forms the young characters as they live the adventure.
Not to mention diversity.