Mobile phones - at what age should a child be allowed one?

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OK so i may seem old before my time with this one, and i don't have kids but i always find it really strange seeing 7-8yr-olds with mobile phones! I was given my first phone at the age of about 13. And it was a brick, that i only had £5 a month for it. I felt so proud that i was given the responsibilty of my own phone. Now i see all these really young kids who have mobile phones, and who's parents have the responsibilty of footing the bill that comes in from all the text messages, and phone calls, from people they've rung who live 2 minnutes away or they see all the time! What age do you think is appropriate to give a child a mobile phone and who should have the responsibilty of it?