  SGR 0418+5729 is a transient Soft Gamma-ray Repeater which underwent a major
outburst in June 2009, during which the emission of short bursts was observed.
Its properties appeared quite typical of other sources of the same class until
long-term X-ray monitoring failed to detect any period derivative. The present
upper limit on $\dot P$ implies that the surface dipole field is $B_p\lesssim
7.5\times 10^{12}\ {\rm G}$ (Rea et al 2010), well below those measured in
other Soft Gamma-ray Repeaters (SGRs) and in the Anomalous X-ray Pulsars
(AXPs), a group of similar sources. Both SGRs and AXPs are currently believed
to be powered by ultra-magnetized neutron stars (magnetars, $B_p\approx
10^{14}$--$10^{15}\ {\rm G}$). SGR 0418+5729 hardly seems to fit in such a
picture. We show that the magneto-rotational properties of SGR 0418+5729 can be
reproduced if this is an aged magnetar, $\approx 1\ {\rm Myr}$ old, which
experienced substantial field decay. The large initial toroidal component of
the internal field required to match the observed properties of SGR 0418+5729
ensures that crustal fractures, and hence bursting activity, can still occur at
present time. The thermal spectrum observed during the outburst decay is
compatible with the predictions of a resonant compton scattering model (as in
other SGRs/AXPs) if the field is low and the magnetospheric twist moderate.
