Logan has a very strong sense of identity that his role is being the baby of the family. He loves seeing baby animals in books / movies. After watching "March of the Penguins", he pretended to be a baby penguin for months and would hide under a blanket and jump out of his "egg". He has cycled through being a baby seal / dog / bird / panda / turtle, and come full circle to be a "baby human".
I'm partly to blame since I'm always telling him "don't grow up Logan, stay a baby forever". Now he has this fear of growing up. He has even claimed he's going to stop eating so he doesn't grow any bigger!