I taught a set of twins to piece & they already knew how to rotary cut. They were probably 11 or 12 at the time of their first lessons. The girls had been crafting for years, used to make those no sew baby blankets and cut their own regularly. Both were very careful and had not had any injuries from it, however, I precut their fabric in a kit form and let mom know she can work with them to cut fabric at home if she wanted to. I didn't want to be responsible for the rotary cutting. I demonstrate using shears only, even for adults....if they specifically ask for rotary lessons, I'll do that but otherwise, I keep it simple.
Kids 6 and up are able to piece. Utilizing precut charms, they can easily make 4 patches to learn to sew with a 1/4 inch seam allowance, press and line pieces up properly. My daughter LOVES piecing and that's how she learned at 6. Since the twins were older, they each had very different ideas of what they wanted to learn. One preferred embroidery and applique and the other only wanted to piece. Simple strip piecing only lasted a lesson or two before both girls were ready to move on.