I think I was 14 when I got a mobile phone for the first time. I recall that it was totally unexpected and was more or less only for people to get in contact with me when I was out.
I'd generally say around 14-15 seems like the ideal time to get a mobile. Like ScarySquirrel said; phoneboxes are there for a reason. Children need to learn some independance and common sense at some point. How many youngsters these days even know what phoneboxes are?
At the same time though, if the mobile was perhaps an old model, on PAYG, etc. I wouldn't worry too much about their age. For newer, flashier models though; I think they can have one of those when they can pay for it themselves.
The whole discussion, is of course subjective and it really should be based on the particular child and parent concerned. If the parent feels they've earnt it, etc. rather then mere concern -- then they're ready.
I was never spoiled as a child and I'll be damned if I ruin any kids of my own (if and when). It seems (and I don't mean to drift off on a tangent here) that kids these days seem to get anything and everything they want thrown at them. I mean, I'm a twenty-something year-old bloke and I've seen kids half my age (if that) with things I've wanted for a long time but not had the money. It seems that materials these days are expected rather than earnt. I recall that when I was younger, when my brothers and I got a console we were extremely greatful. Sadly, I don't see many children behaving this way anymore.
The question is; how do you raise a child today without spoiling them and making them feel hard-done by? Such is the polemic of an affluent society I guess. It seems simple to just not give into children's demands, but a lot of the commodities today are much cheaper than when we were younger, it becomes difficult not to. Somehow, you have to teach children the virtues of patience, gratitude, etc. Nietzche, I'd imagine, would be turning over in his grave.