While no one seems to know when the best age to give a child a cellphone/smartphone is, a recent article entitled When kids get their first cell phones around the world takes a snapshot of how parents in different countries seem to have answered the question. Cost certainly plays a big factor, but US parents seem to be more willing to give their children phones at an earlier age, starting at age 6. If you are not familiar with the recommendations of the American Academy of Pediatrics on media use, you can find a write up on their site.