  The recent discovery of the "weak field, old magnetar", the soft gamma
repeater SGR 0418+5729, whose dipole magnetic field is less than 7.5 \times
10^{12} G, has raised perplexing questions: How can the neutron star produce
SGR-like bursts with such a low magnetic field? What powers the observed X-ray
emission when neither the rotational energy nor the magnetic dipole energy are
sufficient? These observations, that suggest either a much larger energy
reservoir or a much younger true age (or both), have renewed the interest in
the evolutionary sequence of magnetars. We examine, here, a phenomenological
model for the magnetic field decay: B_dip} \propto (B_dip)^{1+a} and compare
its predictions with the observed period, P,the period derivative, \dot{P}, and
the X-ray luminosity, L_X, of magnetar candidates. We find a strong evidence
for a dipole field decay on a timescale of \sim 10^3 yr for the strongest (\sim
10^{15} G) field objects, with a decay index within the range 1 \leq a < 2 and
more likely within 1.5\lesssim a \lesssim 1.8. The decaying field implies a
younger age than what is implied by the spinown age. Surprisingly, even with
the younger age, the energy released in the dipole field decay is insufficient
to power the X-ray emission, suggesting the existence of a stronger internal
field, B_int. Examining several models for the internal magnetic field decay we
find that it must have a very large (> 10^{16} G) initial value. Our findings
suggest two clear distinct evolutionary tracks -- the SGR/AXP branch and the
transient branch, with a possible third branch involving high-field radio
pulsars that age into low luminosity X-ray dim isolated neutron stars.
