Why did Mohammed have different marriage rules than other believers?

[SisterJennifer]

Thanks for the answer. Is it also true that he married a six year old girl and consumated that marriage when the girl was nine? Is this a common practice or something that was exclusive to Mohammed?

It's not something exclusive to the Prophet. Prophet came to set the example for all of mankind. And you will find the same practice was in effect in many parts of the world (including Europe) at this time.