Hi, @Leah---Tom0. Age discrimination will get you in a load of trouble. Over six years, my pickier guests have been of every age, and you may find that, as well, as you continue to host, so that your "cheap old people" and "cool young people" line blurs. I can suggest, though, that if the breakfast thing is becoming a problem, make a note about it in your description or rules.
"A note about breakfast: We provide the eggs, bread, coffee, ...., and you are the chef!"
Repeat it in your initial message when a guest books.
As to your last question, your response to Doris's written review is doing you no favours. No one knows she gave you four stars, and they don't need to. You have made yourself look defensive. Understandable. I hate four stars, too. We all do. But Doris will never see it. Keep any responses professional, and always resist the temptation to lash out.
I get you about the price. Like you, I keep my price low because of amenity limitations and wanting it to be affordable to the college crowd. I wouldn't raise it. Just maybe be clearer about breakfast, and see if that helps.