Each day there is a new development in babies growth. Each day they learn a new skill. By three to four months of age babies try to explore everything with their mouth - from their own fingers to almost anything that they can lay their hands on like hair clay crayon toys etc. This habit of mouthing will most often continue till they are two and in some cases even beyond two and is often a part of the natural developmental milestones. Most often the habit resolves on its own. However some kids continue the practice beyond their developmental stage.