"The golden age" of coloring books is believed to be 1960-s, because almost all characters were depicted in them, as well as some products (like Planter Peanuts or Campbell Soup)
Basically coloring designed for children from 4 years. It is necessary to offer the child a few options for the different colorings. It happens that some of them are out of childâ€™s interest, while others will love it. So try to pick up the coloring in accordance with the interests of your children.