What's the right age for a child to have a mobile?

Every week a Guardian Money reader submits a question, and it's up to you to help him or her out – a selection of the best answers will appear in next Saturday's paper. This week's question My daughter starts secondary school soon and wants a mobile phone. I'd rather hold out for longer, but she insists all her friends will have one and it's something she needs. If we do have to buy one, what do we get – contract or PAYG? What advice can other parents pass on? What are your thoughts?