My son was, he had an umbilical hernia, which basically means that the whole that the umbilical cord feeds the baby through never fully healed, and air would get in there and make it stick way out, so he played with it constantly, it eventually fixed itself and he doesn't bother it anymore (he is almost 9) I have a friend though, her daughter will be 7 in Sept and she still plays with her belly button, when she is tired or upset she constantly sticks her finger in there, no idea why, or how to stop it sorry.