Graphing calculators allow you to graph functions. Learning to use them is a valuable skill that a child could do whenever ready to use one.
I should clarify my previous response. I think it's valuable to keep in practice doing basic mental math (e.g., there is no reason to need a calculator to convert meters to millimeters). Simple addition, subtraction, multiplication, and division can be done quickly and easily and I wouldn't want my child to get out a calculator every time she needed to use those.
For complex things, like graphing, exponential functions, etc., then a calculator can be very valuable as long as the underlying concepts are understood. I don't think age matters.
What concerns me is that I often see students who struggle with very basic equations when they don't have access to a calculator. I'm sure they learned these skills at some point, but they just have become very reliant on calculators (and maybe weren't confident enough in math in the first place, but I don't know). I think there is a benefit in using mental math regularly for these relatively simple calculations to keep a confidence and facility with it.
I think kids who are really confident with math find it quicker and easier to do these simple calculations in their heads anyway versus using a calculator; I'm definitely not talking about working out complex problems when much more can be accomplished using a calculator.