I'm not sure what you mean by an educational psychologist, but there are a few answers to your question that could apply. First, some psychologists have a degree that's abbreviated "Ed.D." which is technically a doctorate in education but these folks can be licensed as psychologists with further training and can then do all the same kinds of work as any other clinical psychologist, with any age group of clients.
If you're talking about a psychologist who specializes in educational assistance, this would usually mean a therapist who has special training in things like testing, learning styles, developmental paths, social/school development, and making educational accommodations for students who are having trouble. This kind of psychologist could work with students of any age from nursery school through university.