Don,

The magnetic field leaves an imprint in the rocks on the planet's surface. The NASA spacecraft Mercury Messenger as its orbit decayed was actually in low Mercury orbit, and could measure the residual motion in the rocks of the planet's crust. The conclusion is that the magnetic field is actually older than had been previously thought, about 3.7 billion years old. In 2017 the European Space Agency will launch a probe, BepiColumbo, which will make more precise measurements and will measure more precisely the age and strength of the magnetic field, which, like that of Earth, results from the dynamo effect of the iron core of the planet. But its 3.7 billion years old, to within 10%.

http://earthsky.org/space/an-ancient-magnetic-field-on-mercury

The idea that its less than 6000 years old is pure nonsense.