There is another advantage of colorings for adults. Most of them are very nice and include impressive black-and-white contours that can be turned into a work of art. Imagine the feeling of satisfaction that you get when you have finished drawing and will be able to appreciate the beauty of the masterpiece created by you personally.
In 3-4 year old kids are more confident in coloring. They're not trying to draw freely, and act within the circuit, trying to repeat the figure bends.