My youngest student has been 7 or 8 (she wanted to make her grandmother a lap quilt for her 90th birthday). I've had 3 children start at that age. I dig out scraps with straight lines and have them practice on my featherweight until they can keep their sewing where it belongs. I have had some start and stop cold with a quilt half done. Wish the mamas would encourage them to continue. But one grandson came back from a year abroad (he was now 15) and came up to where we live to "get in touch with his artistic side again". He made a wonderful quilt for his grandfather on the other side who had had a stroke. It turned out beautiful! Much brighter than I would have done, but when it was done it was really a winner. The family near the grandfather said he hugged it and cried. He lives across the world from us.
By the way. One way to help a child understand the flexibility and feel of fabric is to give them your smaller scraps for making posters. We had several G-kids with us for several weeks recently and they made a (1) fathers day banner for their grandfather, (2) a "Welcome Daddy" banner for their dad who was coming from Alaska to take them home, and (3) a "Happy 12th Birthday" banner for a cousin who couldn't be here for the 4 weeks but wanted to share her birthday with them (a 3 hour trip each way!). The banners were outstanding! I have a whole lot of 2" squares from water color quilts I did some years ago and they glued those around the edges. Something of which we all were proud!