I started letting my son use a calculator at age 9 with some of the Singapore Challenging Word Problems.
I did it so he'd be more comfortable with it when it was allowed on the Explore exam.
If the problem is basic computation, he can't use it.
If the problem is checking problem solving, I'm willing to let him use the calculator so a computation error doesn't mess up the whole problem. So, with the CWP, if it's in the challenge section he can often use the calculator.
For high school courses and some college courses, the TI-84 is currently the calculator of choice in many places. That's changing some now with the N-spire (also TI). Graphing calculators are cool because of some of the investigation they allow students to do. The drawback is that these calculators most often aren't used effectively. Our cc has a graphing calculator as a requirement on the departmental syllabus in Intermediate Algebra. I don't require it when I teach because I think everything in that course can and should be done with at most a standard calculator. But in later courses they want students to have some familiarity with graphing functions on the calculator and using some of the features (calculate intercepts, points of intersection).
I expect I'll let my son use a graphing calculator on occasion in high school (show him how to do certain things on mine), but I don't think I'd get him his own until calculus...or maybe when he went to college and got whatever was required there.
So I'd get the TI-30XIIS (specific model...two line calculator so you can see what was typed) to get him familiar with notation on the calculator, but I'd only allow its use on word problems and if a decimal approximation is needed (like with finding approximations from the quadratic formula or playing with calculator entry for radicals).