I think our lifestyles are so different than what they used to be when I was growing up. I got my first mobile phone (w/ limited features) when I had my 13th birthday. I'd only use it to text family or call them. There are plenty of services that allow you to give your child a phone and will only let them call emergency numbers or you can program it to call mom or call dad, etc etc. Six years old is really young in my opinion, really a child shouldn't need a cell phone being that young, sure they go out on field trips, they might go over to a friends house, but all of the places they go, there should be a phone handy there in case the parent needs to call or the child needs to call. I'm a bit iffy about a 6 year old having a cell phone.. maybe a bit older.
I understand what you are saying and agree with you for the most part except the bolded part. When on field trips, sure there is always a grown up with a phone, but what if god forbid there is an accident or something bad happens? The likelihood is that you will not be the first person that adult calls, even if your child is directly affected or injured. There's also a possibility that the grown up doesn't even have the contact details of all the parents, especially if the field trip has a large group of children. I'd just like to know that if my child was able to call me in the case of an emergency that they would. For me it's purely for keeping me in the know and a safety measure. Like most posters said, my LOs having phones will depend greatly on their activities outside the house, whether that's 6 years or older.