Driving a hire car across America under 25?

Many US car rental firms do not allow the rented car to leave the state.

You may take the car across the US if you have the permission of the rental company to do so.

Be very aware of the rental agency's policies concerning such a trip. You may get hit with very expensive mileage charges and, of course, you need the permission of the rental company to take the car out of state.

I never had a problem ..I have driven through 36 states ...loved them all ..and all different ..once you have the car there should not be a problem ..but driving is very different ..they drive slow compared to uk ..god help any American who tries to drive in Italy ...every day a Daytona 500 ....ask the hire company if they have any out of state cars they want returning ...I had a mustang that I drove from seattle to Miami ..got it at economy rate

I believe only NY and CA have laws saying rental companies must rent to anyone over 18 (unless they can prove some insurance hardship or somesuch). The law does NOT say what limits can be put on the rental or how much the company can charge. You will likely pay and under 25 daily charge. In addition the rental company may limit mileage AND may limit the radius of taking the car, often limiting that to within the state. You may also be charged a fee (varying but usually LARGE) to drop the car at a place other than the rental origin.
BEFORE you book your trip make absolutely certain you are clear on the rental charges and limits. Assuming you find a rental company that will rent and you can afford, you may want to familiarize yourself with the rules of the road in the US. Look for the Drivers Handbook from the department of motor vehicles in your arrival state.

It doesn't matter what states you drive in -- the restriction is from the rental car company, not the state government. So if you could legally rent it from the company, you can legally drive it anywhere that they allow (be sure to study the policy -- most will not let you cross national borders into Canada or Mexico.) Also make sure you are aware of mileage limitations. Make sure your rental is "unlimited miles". Some rentals only allow a certain number of miles per week, like 1,000. If you are driving across the US especially out West, you can drive that far in less than 2 days and you pay a heavy price when you return the car if you have exceeded the mileage.

Rent a car. Hire a car would mean a driver and the car in the USA.
It doesn't matter what age the state says you can rent a car, it is what the rental car agencies say. I don't know of any. Car rental companies that will rent to anyone under the age of 21.
Once you rent a car you can drive it anywhere. The age to rent the car has nothing to do with what state you are driving it in. As long as you have a valid drivers license and car insurance.
Mrsjvp is correct. You will have an extra fee for renting under the age of 25 and a large fee if you return the car to a different location from where you picked it up.
Reputable car rental companies
Enterprise
Avis
Alamo
Hertz

every car rental agency runs things differently you will pay OUTRAGEOUS extra fees because you are under 25. you can rent a car for unlimited miles, but again you will pay OUTRAGEOUS fees to drop it off in a different spot than where you picked it up

no one is going to rent to an 18 year old. maybe 21.

if you do not have a valid international DL, you won't be renting a car in the US