My feeling is if they are interested they are ready. If the attention span is five minutes then deal with it in short spurts. And let them just "help" you to begin with.
I remember sitting on my moms lap at the sewing machine when I was probably about 4 years old "helping" hem dish towels for my grandmother. Then I eventually graduated to using a needle and thread to hand stitch doll clothes for the precursor to Barbie.
When I was 13 I made my first garment for myself, a pair of Bermuda shorts. The zipper wasn't done exactly right and they were a bit too big but I wore them around the house all summer.
By the time I was in college I was making about half my clothes. And anything that was for a special occasion.
Wish I had been a bit more patient when DD was initially trying to sew as a young teen. I think I discouraged her unintentionally, criticizing her crooked seams.
She's beginning to show interest again and I got her a machine for Christmas. She hasn't really used it yet, but its there when she wants to use it.
Am thinking about ways to let DGS become more involved than just picking out fabric, which he has been doing for awhile. He is very observant and knows pretty much how things are supposed to be done but also very short attention span.