The Earth's Age

Some people believe that the bible proves the earth is only 6000 years old. They deduce this by counting back from Jesus' day assuming a generation for each begattification. However, as Methuselah lived to 900 years we can assume that a normal lifespan is 600 years (people lived a lot longer in those days!), making the earth actually 180000 years old -- a much more believable age!