How long do you have to wait to drive with a passenger at 18?
In the UK you do not have to wait or be 18, you need to have a full licence or have someone with you with a full licence
At a minimum, a Class C Commercial Drivers Licence with Passenger endorsement and no air brake restriction (if the vehicle is equipped with air brakes). If the vehicle has a GVWR of over 26,000 lbs (which is a bit unlikely with an 18 passenger vehicle), then a Class B CDL would be required, again with the Passenger endorsement and without a restriction on air brakes, if the vehicle were so equipped.
Do you have to have a passenger over the age 21 if your over the age 18 an knows how to drive with a learners permit?
In some states, yes. In other states, the licenced passenger may only have to be 18. Some states may also require that the licenced passenger have held a drivers licence for a certain period of time. You need to inquire with your state's Department of Motor Vehicle for a definitive answer as to what the laws are for your state.
By the way this is California state law. It states,"Once you receive your license you have to wait a year(or until you turn 18) to drive 11PM-5AM/drive minors with out a 25 aged licensed driver." The "or until you turn 18" is the same as waiting a year. So if you have your license, and you're 18, you can drive between 11PM-5AM and the best part, driving around other people under 20 aka friends/family/minors.