I have the same basic criteria for everything like that:
- Did they ask?
- Have they demonstrated an ability to handle the responsibility?
For a phone, the second point would be demonstrated by showing they can take care of other prized possessions, i.e. not leaving them where my daughter can run them over with her wheelchair, etc.
The other factor to consider is the policy of your child's school. Assuming schools over there are as annoying as here, some have downright draconian rules on the subject. Make sure you're not setting him up to get it confiscated.
Myself and my three siblings were all teenagers at the same time, and we were all heavily involved in jobs and extracurricular activities. Every Sunday night we sat down for a "calendar meeting," where we copied events from each other's personal calendars, coordinated rides and meals, and planned flexible events like what day to shop. That meeting never took less than an hour, and sometimes took two or three. It was a huge source of contention, but also unavoidable.
The reason I tell you this is now that my wife and I have smartphones, our calendars are always synced with each other. This is so much nicer that I can't imagine not wanting my kids as part of the same system. Also, there are things like family chat channels and GPS tracking apps that would be useful. Obviously that opens up some potential concerns, and we might have a dumb phone as a backup in case of rule breaking or whatever, but I think in general the benefits would outweigh the risks.
Also keep in mind you don't necessarily have to go straight from no cell phone to unfettered access to one. Maybe at first it can be restricted to only calling parents. Maybe you check it out to him like a librarian only for certain occasions until he proves himself. Just make sure your rules and their consequences are extremely clear.