Since the field probably started when the earth was formed, the present rapid decay of the field is strong evidence for a young earth.
Old-earth proponents, however, correctly point out that the earth's magnetic field has not always decayed smoothly.
(not well known at the time) showing that the overall strength of the earth's field has indeed steadily declined by about 7% since 1835, when it was first measured.
The decay rate depends on the electrical resistance of the earth's core, and the observed rate is consistent with the estimated resistance of materials at core temperatures and pressures.
In 1986, Voyager 2 verified the Uranus prediction, and we should find out about Neptune in early 1990. After creation (and the Fall), the electric current in the earth's core would decay slowly, as would the field, for 1656 years, until the Genesis flood. Instead, it would have a more complex shape, with strong "higher-order" components: quadrupole (four poles), octopole (eight poles), etc. "An analysis of the earth's magnetic field from 1835 to 1965," ESSA Technical Report IER 46-IES I (July 1967) U. Paleomagnetic data confirm the existence of such components in the field in the past. The resulting initial field strength is consistent with the present geomagnetic data and a 6000-year age for the field. Humphreys is an ICR Adjunct Professor of Physics and a physicist at Sandia National Laboratories, Albuquerque, New Mexico.Thus we have a plausible explanation for how the current in the core got started. The Laboratories have not supported this work, and they neither affirm nor deny its scientific validity.