I relate this question to cooking. Dangerous stuff, but taught with the respect for safety and what the machines/stove, oven, etc do, it's all a matter of when the child is ready. Child #3 for me was "sewing" fabric on my Viking at the age of 4. She also loved poking in and out of a design she made herself on muslin in a hoop in her first versions of embroidery.
My eldest is 27 and I'm pretty sure I'd have to be much more careful with her
