It is believed that the company The McLoughlin Brothers (en.wikipedia.org/wiki/McLoughlin_Brothers) invented the coloring books. They published a book called The Little Folks' Painting Book in collaboration with Kate Greenaway in 1880.

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.

Mega Ex Pokemon Coloring Pages