I'm not sure exactly, but based on the Amazon order history, it was a little after their 4th birthday. At that point, attention span is low, so you have to do f

I'm not sure exactly, but based on the Amazon order history, it was a little after their 4th birthday. At that point, attention span is low, so you have to do far less than you think is even worth opening the book for and then switch to something they enjoy more like mommy/daddy reading to them (which we started way, way earlier, of course).