I was using a rotary cutter when I was nine. My parents figured I was better off learning how to do it the right way with their tutelage than to half to try to mess around and give up on a project. (They also taught me to use the "real" stove when I was six. I was using it unsupervised on a regular basis by the time I was eight.) It depends on the kid - are they careful and focused, or are they easily distracted and klutzy? Do they regularly have small accidents with other stuff, or are they very careful? I was far more safety-conscious about stuff than my peers.