i don't know how old i was when my mom started teaching me. i know i was so young that i now can't remember. i had my own very real sewing machine by the time i was in kindergarten.
every stitch i sew to this day reconnects me to mom. sometimes i'm not even here. i'm waaaaaaaaaay back then in our sewing room.
short answer - if they ask to be taught they are old enough to learn. ;-)