I think it would be a better thing for 10 year old to have mobile phone. No days mobile phone are so advanced that it would make the children smarter in some ways. Not to forget a emergency they can allways call their parents and let them know of a current situation. Lets say a child gets lost at a concert and need to call someone for help. In these situations it would help them alot.
Unless you are talking about a phone used for only calling, then no. Children are not responsible enough to use it, nor should they even have a need for it. When they get it, they don't know how it works. It is a quasi-unknown and advanced device, almost magical to them. If they know how they works, and are knowledgable enough on how to use it, when to use it, why to use it, and to be responsible using it, and if they put their school and education above all else, then yes they can have it.
Of course, that is for the few, the majority of children should not have them.