Countries where under 21's can hire a car?

[zasterguava]

I want to go on a road trip next year abroad, but, can't for the life of me find somewhere suitable that allows under 21 year olds to hire a car. Ideally, I want to drive accross the US, but, again you have to be over 21. I read there is some sneaky way you can get round this by insuring it to a over 21 year old and claiming they're the passenger but that doesnt sound promising.

I have a Valid UK driving license and hoping to find any country that allows under 21 yo's to hire cards.