I want to travel from SF to Las Vegas and spend 4 days in LA in the middle. Well I want to rent a car to do all the path, but the following question came to my mind:
What is the minimum age to rent and drive a car in California and Nevada? I am 24.
As @waiwai933 said, and it's applicable to most states in the US, you need to be 21 to rent a car. SmarterTravel website summarizes the issue pretty well. What you need to know is that you'll have to pay $25 to $35 a day as Young Driver Fee at most car rental agencies, with a maximum of $300 (so for rentals over 10 days you don't pay any more for the rest)
Watch for good deals though, I remember I never paid it at Hertz because they often have a discount for young drivers. It's written that it's until a certain date but I used the same coupon number a year ago, so they probably extend it every couple months.
If you are under 21, read SmaterTravel article, as they say there are few offers but it becomes very pricey.
If you don't rent, I don't know what's the law in America. 16? 18?
By the way, a useful fact is that in California liability insurance is not mandatory. It means if you don't ask for it at the time of rental, if you hurt/kill someone in an accident, you'll have to pay all of the hospital fees.