Let's put a stop to a common misconception apparent in this thread.
Eggs have an air cell at the big end of the egg. As an egg ages, the contents of the egg lose moisture through the shell due to its porosity and the yoke and white of the egg shrink. However, the air cell expands as air is taken in from the outside, again due to the shell porosity.. The size of the air cell is indicative of the age of the egg as an old egg will have a larger egg cell and consequently float when submerged in water. Newer eggs have a smaller egg cell and consequently sink.
This has absolutely nothing to do with the formation of hydrogen sulphide. Air alone determines whether an egg floats or sinks so i'm afraid the information provided by bhg481 is incorrect.
Hydrogen sulphide and other gas production in an aged egg has nothing to do with it. You can crack an egg that floats without experiencing the whiff of hydrogen sulphide.