Yes, this issue has come up before (annually I think) regarding the age to drive on Corfu. If one looks through various car hire companies on the island, you will find that the age of hiring is between 21 and 23 years of age.
Not sure on the ruling of how long a full licence has to be held to hire a vehicle, but I have heard the term, one year before.
Helmets, another favourite subject of the boards. I agree, if a helmet is given on hiring a scooter, quadbike, etc, then you are required to wear it. If you are not offered one, then ask for one. It makes sense on the roads of Corfu, and any other islands to wear a helmet. You will find threads on the boards of horrific accidents where the injuries could have been reduced if the helmet was worn.
Safe driving, what ever the vehicle.