Seems like there was a lot more science fiction written for the young reader when I was a kid then I see now. Back in the 1960s you had the big names of the time writing stories aimed at young readers. Writers like Heinlein, del Rey, Asimov, Clarke. All wrote to that age group. If you can find them the Winston juvenile science fiction stories are great. Andre Norton wrote over 100 books most of which would appeal to young readers.