I taught my son at the age of 9. He was really interested and I took that as the que. He wouldn't have been ready before that. In general, girls have better fine motor skills earlier on than boys. My son was pretty standard!
We went over safety first. He picked out fabric for a Ravenclaw inspired quilt. It was a four patch. I used a speed-control machine. I let him try all the fancy stitches on scrap fabric first.
Funny story: So he learned to sew but after the quilt, lost interest. One day after school he asks if he can use the machine and make a bookmark out of Pokemon fabric. He had already made one for himself, but I thought he lost it. I set him up, off he went, finished it up, and I pressed it for him. The next day he has this rubik's cube. Upon questioning and investigation, he tells me that his friend liked his bookmark and offered to trade his rubik's cube for it. DS said he make him one and they made the deal the next day. Score one for handmade crafts!