We calculate the coupled thermal evolution and magnetic field decay in relativistic model neutron stars threaded by superstrong magnetic fields (B > 10~(15) G). Our main goal is to evaluate how such "magnetars" evolve with time and how field decay modifies the transitions to core superfluidity and cooling dominated by surface X-ray emission. Observations of a thermal X-ray spectral component and fast timing noise place strong constraints on the presence of a superfluid core. We find that the transition to core superfluidity can be significantly delayed by magnetic field decay in the age range ～10~3-10~5 yr. The mechanism of Hall drift is related to the stability of the core magnetic field and to currents flowing outward through the crust. The heating effect is enhanced if it is continuous rather than spasmodic. Condensation of a heavy element layer at the surface is shown to cause only modest changes in the outward conduction of heat.
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