here is how it works in Utah:
15 yrs 6 mo- Driver's Permit (You can drive aslong as you have a parent or adult over 21 in the passenger seat)
16 yrs- Driver's License (You can drive by yourself, no supervision, but no friends in the car unless you are going to or from school, church, etc. You can't drive past midnight too, i believe)
16 yrs 6 mo- (You can drive with friends in the car, I'm not sure about any restrictions)