as soon as you are old enough to spell simple words, mainly 16, although 15 yo's Saturday jobs are a doddle, it would get you ready for grown up writing, filling out contracts, have to be spell perfect, a couple of apostrophe's wouldn't go amiss either, but go ask, you have a fifty fifty chance of getting the job, make sure you know what the wage is before hand then wage negotiation is a little easier, ask a couple of friends what the Saturday rate is, dress smart and business like.