it is less dense than water.
Well an old egg will have more air inside of it.An egg has roughly 14000 incredibly small(not viewable by the unaided eye)openings that allows the embryo to take in air to live as it needs oxygen. The USDA considers a 4 week old egg "fresh" ! However an egg has an air space in it which grows ever so slightly each day. Therefore the older the egg, the more air space, and the less it weights. The old eggs will then float, inccedently this is how you can tell if an egg is fresh. Fresh eggs sink ! The older an egg is, the higher it will float in a body of water. If near the top, or floating, bad egg. I wouldn't eat it!