As far as I know there is no age restriction to join Steemit. However guidance from the adults is an absolute necessity to protect them from online predators. In our part of the world we have #myjuniors initiative to help Steemians kids to learn to create content. We held simple workshops (drawing/crafts etc) for these children (once a month) and encourage them to mingle with other children to build friendships. The parents can choose to create a Steemit account for their children or they can use their parents' account to post. My kids are using my account to post their artwork once a week.