Perhaps oddly, I never read the Heinlein juveniles in the early ’70s when I was a juvenile. I read Norton, Nourse, and adult novels by Asimov, Clarke, and Simak, and John Christopher, but the only Heinlein I remember from my early reading of SF is Orphans of the Sky, which made little impression on me. Even a few years later, the only juvie I read was Tunnel in the Sky.
So all I can say is that when I did make a point of reading them, about ten years ago I guess, I thought them immensely entertaining. I can’t speak for “kids these days” (my daughter read Brian Jacques and Tamora Peirce and Charles de Lint, my son not much at all, alas) — but on the face of it I would think Heinlein’s work likely still appealing.
(I will say that my brother — 10 years younger than me — did discover RAH’s juveniles, and read them with enjoyment. But as he went from them to Battlefield Earth. I’m not sure what to make of that!)
Read my first Heinlein juvie in my 50s.
By the time I was 12 or so, I was so done with the Children’s section (don’t get me started on Teen books — hot rods or baseball for boys, prom or bridesmaid for girls) that I was reading Harold Robbins and John D. MacDonald.
The first SF novels I read were Andre Norton’s “juveniles,” but I jumped to Heinlein’s “juveniles” fairly quickly because I thought they were just more complex and interesting. As Gary mentioned, one factor in this was almost certainly that at the time (late ’50s, early ’60s) Norton and Heinlein’s juveniles were among the few SF novels of any sort available in the library, especially the school library (where the librarian scolded me for reading “that mind-rotting junk” every time I checked an SF book out).
I think that the Heinlein juveniles still have a lot to offer to young readers that they’ll find interesting, entertaining, and maybe even useful; certainly all the stuff about young people going out into the world and trying to make a life for themselves there, battling the odds thrown against them, is just as germane as it ever was. The question is, are these 60-year-old books too dated to allow modern readers to get through to the universally applicable stuff? Once you get beyond the “current-day” on-Earth stuff in a book like Have Space Suit, Will Travel and out into the solar system, I’m sure the kidnapped-by-aliens adventure plot, with eventually the existence of the Earth itself at stake, would appeal just as much to kids today as it ever did. Would they be willing to keep reading a book, though, where the “current-day” stuff is so different from the world they live in themselves, where there are no cellphones or personal computers or Facebook, where kids work as “soda jerks,” something that most of them have never even heard of? Will the fact that most of these books start out in a “present-day” that’s clearly the 1950s rather than the world that kids today are familiar with make the present-day part just too dated and alien from their own experience for them to be able to get through it to the Sense of Wonder stuff that eventually follows? (THAT stuff is still pretty good, even when compared against today’s “adult fiction.”)
I don’t think kids have any trouble reading stuff set in different worlds, and I’d consider this another example of one. The “current-day” stuff in the Narnia books – or Jules Verne, for another example, since I’m about to head back to SteamCon – never bothered me.