It varies SO much! There are some kids I know who are nearly 3 and still just notinterested. Part could be lack of confidence, but I think at least 90% is just not being interested, and expressing their creativity in other ways.
Keep offering them every now and again - they *must* be full of vitamins or something, since so many kids just want to eat them!! Alternatively, if you can handle it, try paint and thick brushes, or shaving cream to muck around with, or playdough. They're all different ways to make marks and manipulate a material, each as valuable as each other.
Good luck! And don't let the blue book rule your life!