The constant imagination..

From my point of view, we never actually lose our sense of wonder. We give less value to our imagination as we grow old, resulting in the loss of our understanding of wonder. This phenomenon is similar to Lamark's theory of using and disusing organs. His theory states that the organs you use more get more potent with time, and those you use less get less powerful. So, it can conclude that our imagination weakens due to the lack of constant use.