I got into sci-fi because my dad had a giant shelf with nothing but pulp sci-fi books, mostly from the 60's and 70's. Most of them were not very hard sci-fi, but I loved reading through all of them as a little kid. They triggered my imagination, especially the ones that came with little maps in the back where I could follow the story's progress. Jack Vance's Planet of Adventure was one of my very favorites, full of.. well.. adventure (it delivers exactly what it says :)
Most of them were already ridiculously dated, but it was the idea of traveling through space, exploring new places with strange and exciting cultures that made them so appealing to me.
I tried to find more books in the high-school library when I got older but was disappointed to find almost nothing outside of the traditional literature like H.G. Wells. It did motivate me to get into fantasy literature like Beowulf and LOTR to fill out my reading list for English class (English is not my native tongue), anything to get out of reading "proper" literature.

Anyway, if you want to recommend science fiction to young people, keep it simple and trigger the imagination. The hard stuff can come later.