It obviously varies a lot depending on the child, family, and general situation. There are three really big questions I would ask, though:
- How responsible is the child, in general? Does he/she frequently lose/forget items?
- Do you trust him/her to carry technology around without breaking it?
- Do you trust him/her to not use it in some situations (school, movie theater, etc.)?
- Also consider any restrictions at school or extracurricular activities; many have strict no-electronics rules.
- WHY: Is the phone for...
- safety (child can call 911 or adult in case of emergency)
- socializing (child can call/text friends)
- organization (child can call/text to inform adult of schedule changes)?
- Is the child pushing for this phone? If so, what's their motivation? "Everyone else has one" isn't as compelling as "I want to be able to let you know swim practice was cancelled," in my opinion.
- WHAT: Consider the "level" of cell phone.
- A smartphone, for example, has a lot of apps which can significantly increase the distraction factor (and, potentially, expose personal information such as home address, current location, and so on).
- If this is just an emergency/general use phone, an inexpensive flip phone should be all that's needed.
From personal experience: We have a cell phone that "belongs to" our daughter, who is ten. It's got a plan that restricts contact numbers on both incoming and outgoing calls (currently she can call both parents, a close family friend, and 911), and is intended for emergency use only. She cannot use it (or carry it around) without permission, and must ask to even get access to it. She's not terribly good at self-regulating its usage, and in one instance set a "walk the dog" alarm for 4am which made the phone start barking. While that was pretty annoying at the time (although funny in hindsight), it did give us a chance to have conversations about what a phone is really for.
Once she develops some more personal maturity and responsibility, I plan to let her carry and/or use it more often, and open additional numbers (e.g., to chat with her own friends in addition to being able to phone for help) -- with the warning that she'll have to chip in if talk time or texts start exceeding a set threshold.