1963’s Podkayne of Mars was, if Heinlein’s comments in Grumbles from the Grave can be believed, not intended as a juvenile:
March 10, 1962: Robert A. Heinlein to Lurton Blassingame
Is Poddy a juvenile? I didn’t think of it as such and I suggest that it violates numerous taboos for the juvenile market. It seems to me that it is what the Swedes call a “cadet” book — upper teenage, plus such adults and juveniles as may enjoy it — and the American trade book market does not recognize such a category.
Despite that, some people, including me, lump it in with his juveniles because the lead is a fifteen-year-old girl, with her eleven-year-old brother in an important supporting role.
Remember how in my review for Have Space Suit — Will Travel, I said:
As I closed the cover of the last true Heinlein juvenile, I really wonder what this book would have looked like if in 1958 Heinlein had been able to envision and publish a juvenile with a female lead.
We will never know the answer to that. We do have the answer to the question what would such a novel look like if Heinlein wrote it five years later and the answer is “horrible”.