One SF author I really enjoyed at that age was John Varley. "The Golden Globe" is still one of my favorite SF novels of all time, "Steel Beach" close behind.
A military SF series I enjoyed around that age starts with "Midshipman's Hope" by David Feintuch.
Greg Bear was another one of my favorites at that age. I started with "Eon".
Maybe all those are hopelessly dated by now! But those are the ones I can recommend from experience.