I think that it would be great for a 3-6 year old. My daughter is 3 and she adores Care Bears and coloring books can keep her busy for quite a while. My niece is 6 and she also loves Care Bears and coloring books. When she comes over we always have the coloring books and crayons ready.
I think that the Care Bear theme would be most attractive to girls under age 6, but kids enjoy coloring in general for quite a while and could probably overlook the theme of the page even if it is a little young for them.