I think that there are a lot of factors into this - but I do agree that it should be based on maturity more than age... I often see middle school kids w/ cell phones and I don't really think that is necessary.
I think that if a child/teen is driving, then their having a cell phone is a good idea, especially if they live in a more rural area. While they obviously need to know it isn't appropriate to use the phone while driving, I think it is a big safety issue for them not to have one period if they drive alone, especially if they drive on more rural roads. Similarly, if your child plays sports and is away on trips (even just afternoon trips a few towns away) their having a cell phone is probably a good idea. It saved me being left at school to wait for a ride when I got back early a few times.
My parents required my sister and I to keep our cells with us when we first started driving, "just in case," and they did pay for our phones b/c they wanted us to have them. If they want extras like a smart phone then I think it's reasonable to expect them to pay.