its different for everyone, because people develope differently.
My growth plates closed in eighth grade, while some peoples don't until senior year.. Or even later for some. I believe (someone correct me if I'm wrong) this is the major factor in if your arm is ready for a slider.
If it feels fine to throw it, than throw it. If it hurts, don't. Just make sure you throw it properly - otherwise it'l come back and bite you.
Dr. James Andrews found that curveballs, when thrown PROPERLY, put less stress on the arm than fastballs. I'm not sure if they tested the slider, but that goes to show how the stigma against early breaking balls may not be very well substantiated.