Under 25 cannot rent anything above Premium according to Avis' terms of use. HOWEVER, you actually can, you just have to get a manager override. In my experience if it's a situation where the only car available is a higher than premium car and I've reserved a premium car, I've had no issue at all. It really just depends on the manager as well as the fleet situation wherever you are. I'm guessing that your previous experiences you may have either been dealing with a manager and not known it or that's just what was assigned to you because of your reserved class shortage.