Anyone supervising a learner driver must be at least 21 years of age and have held a full licence for the category of vehicle being driven for a minimum of 3 years. In addition the vehicle be insured for the learner to drive and also display 'L' plates front and rear. (Not on windows)
Also worth remembering while supervising a learner driver it is just as illegal to use a hand held mobile phone as it is when you are driving yourself. The vehicle must be stationary in a safe location with the engine switched off to comply fully with the law.