SGR 0418+5729: how does a young neutron star spin down to a 9 s period with a dipole field less than 10 (13) G?

The period derivative bound for the soft gamma-ray repeater SGR 0418+5729 establishes the magnetic dipole moment to be distinctly lower than the magnetar range, placing the source beyond the regime of isolated pulsar activity in the P{--}\dot{P} diagram and giving a characteristic age >2 × 107 yr, much older than the 105 yr age range of SGRs and anomalous X-ray pulsars. So the spin-down must be produced by a mechanism other than dipole radiation in vacuum. A fallback disk will spin down a neutron star with surface dipole magnetic field in the 1012 G range and initial rotation period P 0 ~ 100 ms to the 9.1 s period of SGR 0418+5729 in a few 104 to ~105 yr. The current upper limit to the period derivative gives a lower limit of ~105 yr to the age that is not sensitive to the neutron star's initial conditions. The total magnetic field on the surface of SGR 0418+5729 could be significantly larger than its 1012 G dipole component.