It depends on what freedom the seven year old is allowed.
My DD was given one last year, when she was 7... but we live in a small market town and she has a lot of freedom (certainly compared to many on here who say that their 7 year olds don't need them because they never go anywhere).
My DD is allowed to play out in our road (of about 150 houses), the road behind ours (similar size), over to the park and swings. She's not yet allowed to the nature reserve, but some of her friends are. Further, she has about 8 friends who she can call on, and in the summer they frequently move between houses / park, only returning home for tea.
With a phone, I can allow her this freedom, as my only requirement is that she calls me every so often to keep me updated as to where she and her friends are going next.
As a child, I did the same, but I was sent out with some change and used the phone box instead. Without phoneboxes to use, a mobile phone does the same job.
Of course, if I didn't live in an area where she could go out and play, then of course she wouldn't need one... but I would find that sad.