It depends on the child-how well do they follow directions, motor skills. etc.
I wouldn't go younger than 5-6 myself. I think 8 is probably a good age to learn. At this age the can focus more and should have experience following instructions. As far a what's sold as a "child's " machine- They usually do a chain stitch only and just frustrate the child. I would rather wait a bit longer till they can use a real machine.