Regarding cellphones I feel that parents in the best position to make decisions on when to give them a cell phone. Some kids are mature enough to use it at an early age. Some kids are trouble makers and are not ready for the responsibility even in high school. Although sometimes parents do make poor decisions.
Personally, I'd balk at giving cell phones to any kid under ten. I think kids in middle school can start having cell phones. Simple phones just to make calls in case of emergency and basic communication with friends. In high school kids can have some more freedom with phone, but parents need not foot outrageous bills. If high school teens want more minutes, unlimited text/data, smart phones, they ought to pay for it. The only real need is basic calls thats it. Also if a child misuses phones, spends too much time calling/texting and ignores other responsibilities the cell phone can be confiscated. Confiscating a phone is a perfectly fine threat for poor grades, skipped classes and other teenage indiscretions too.