  It is commonly accepted that a neutron star is produced, when a massive star
exhausts its nuclear fuel and ends its life in a core-collapse supernova
explosion. This scenario is confirmed by the detection of pulsars, which are
believed to be rapidly spinning neutron stars, in the central regions of many
supernova remnants. Neutron stars and their associated supernova remnants
should therefore have the same ages. As expected, the age of the Crab Pulsar,
the first to be connected with a supernova remnant (the Crab Nebula), can be
inferred from its current spin period and its derivative and indeed has about
the same age of the supernova remnant, that was produced from a historically
recorded supernova explosion in 1054. However most neutron stars appear to be
much older than the ages of their associated supernova remnants, a puzzle not
yet understood. Another puzzle is that so far no convincing evidence has been
found in favor of magnetic field decay in neutron stars, that is predicted in
most models of neutron stars. Here we show convincing evidence of magnetic
field decay in some young neutron stars, and that the magnetic field decay can
alter their spinning behaviors significantly such that these neutron stars
appear much older than they really are.
