As long as this person is listed in the rental contract as driver, yes. However, if you allow someone else drive the car without been listed as driver nad this person gets in an accident, the rental insurance will deny the claim and you will be responsible for all damages.
It depends on the policy in the USA. In most European countries only named drivers can drive a car regardless of insurance policy - rental cars are the exception for obvious reasons.
The policy for most car rental companies, is that any additional drivers have to be 25 or older. They must sign on to the rental contract at the counter and there is usually an additional driver fee. This fee is sometimes waived for a spouse or a fellow employee if it is a business rental and you have a corporate account with that car rental company.
Swinton Insurance offers temporary car insurance in Pennsylvania so that if you drive someone else vehicle or a rental then you're covered in the event of an accident.
if they're on your insurance
that depends on their insurance policy
Yes
Is the CAR insured? If not, it's not legal.
You can so long as you have their permision.
You can register it but someone else has to drive it.
Anybody who fits the legal requirements to drive in that jurisdiction, and has permission from the vehicle's owner, can drive the vehicle.
For most rental car companies, all drivers must be at least 25 years of age in order to drive the car.
It is illegal to drive in any US state, to drive any car, if you do not have a driving license.