There's an interesting article by Rich Horton in Locus Online about Young Adult science fiction.

What about SF? For many of us, YA SF novels were the "gateway drug" to the genre. In recent years, many people have lamented the "graying" of SF fandom, and some have suggested that there aren't enough good new YA SF books to attract new readers. Where is today's Robert Heinlein? Andre Norton? Alan E. Nourse?

I actually never read any of these writers when I was growing up. H.G. Wells was an early find. "The Man Who Fell to Earth" was another I read at an early age, and it left a strong impression. I cut my teeth on Piers Anthony and Star Trek books. But he makes an excellent point, one that I've talked with several writer friends recently: Where's the good SF for young adults? Why is it all fantasy?

Anyway, Horton highlights four books he thinks are good examples of YA SF, but I think the dearth of good SF for young audiences remains.