The upper and lower front teeth tend to erupt in the mouth around the age of 7. How they decide to come in can determine whether your child will develop an underbite. I have seen many 7-9 year olds come into my office and the parents would tell me, “my child did not have this underbite before and now they do”. Although rare, it is possible that your child has a normal bite when their baby teeth are in, but as the permanent incisor start coming in an underbite can develop.