As you would have figured out by now, it won't be easy and it won't be cheap. Trying to find a shared ride might be a better option if you can find a fellow traveller[s] with a car looking for company and help with the gas. This comes up from time to time in the 'Share the Gas' forum where you could put out such a request, but you do lose your independence.

Originally posted by..Jack Turnbull] Car rental is possible, even as an 18 year old although you will need to use some tricks to keep it affordable and it isn't easy at all. Will PM you some details.
If you have some genuine ways for the younger generation to save money on rentals, it would be great to hear about it in the forums to help others in similar situations and would be much appreciated. Of course if any of the details to the insurers to secure a cheaper deal were incorrect, it would make the Insurance policy void if there was an incident and there could be huge consequences . Be careful and don't cut corners it's not worth it, particularly with US law.