There is an easy answer to this question! Your child is old enough when you need them to have one. A tween is usually in your company or with other responsible adults therefore there is no reason for a phone. If they take a train to and from school you may think it is a good idea to be able to make contact if they or you are going to late. However remember that our parents did not have the technology to do this and it can in fact increase your anxiety rather than reduce it. What if you text them and they don’t respond? It is in their bag, the battery has run out, it is on silent but you begin to worry, the more they don’t answer the more you worry. Do you need this extra worry in your life?
Consider also, that you cannot buy a phone that just makes phone calls so in giving your child a mobile phone you are opening the door to a world that they will step into without your presence: think of Youtube, Facebook, Instagram, Google, Games, all without you being there.
http://www.commonsense.org/ answers all your most Frequently Asked Questions