How can someone over 18 years old learn to drive if they don't have an available adult to teach them?

Go to a driving school, take a course in school that teaches driving, or have the car placed on private property and practice there, where a license is not required.

Get a car and drive around in a field or open space. Get a good feel for it because the only way to become a good driver is to Get OUT and DrivE. Unleash the power within. You can drive a car.

ya. I would just suggest finding out where the nearest driving school is and taking the course. They have driving schools for adults, so you'll be just fine.

search around for a driving instructer. Some are very good and available to work around times that you need them for. BELL driving in particular.

BSM (British School of Motoring)

They'll pick you up for lessons and supply the (dual control) car and insurance cover for the lessons and test. It's easy to organise, they'll help you out with the theory test and all.

Rent-An-Adult.

driving school