In New York State, it is illegal to deny a rental car to renters under 25. But it is perfectly legal to soak them dry. When I was under 25 I rented cars locally a few times. Every rental company except Enterprise wanted to tack on an insane insurance surcharge, even though I had a rental-car insurance rider on my car insurance policy and could decline the CDW. Now I always call Enterprise first when I need a car, even though I haven't been 25 for quite a while.
Also, renting at an airport will generally be more expensive, since there are more taxes and fees associated with rental car sites at airports. But the sites at the airport generally stay open if your flight comes in late....