One of the first Heinlein juveniles (published in 1948) is Space Cadet. It suffers because of that I think. It is the story of a boy, Matt Dodson, who joins the Space Patrol. It’s a kind of solar system police force.
But the problem with the book is how much of it is taken up with the minutia of boot camp and so forth. Maybe young boys of this barely post World War II era were into being soldiers. So this large amount of detailed description of testing and discipline, etc. was interesting to them.
But I grew up in the Vietnam era and the armed forces don’t hold the same attraction for me. I never wanted to be in the military so this sort of thing bored me when I read it.
The second half of the book, when Matt and his comrades go out into space and have adventures, in particular on Venus, is better.
Still, all in all, not one of Heinlein’s better efforts.