  In the standard scenario for spin evolution of isolated neutron stars, a
young pulsar slows down with a surface magnetic field B that does not change.
Thus the pulsar follows a constant B trajectory in the phase space of spin
period and spin period time derivative. Such an evolution predicts a braking
index n = 3 while the field is constant and n > 3 when the field decays. This
contrasts with all nine observed values being n < 3. Here we consider a
magnetic field that is buried soon after birth and diffuses to the surface. We
use a model of a growing surface magnetic field to fit observations of the
three pulsars with lowest n: PSR J0537-6910 with n = -1.5, PSR B0833-45 (Vela)
with n = 1.4, and PSR J1734-3333 with n = 0.9. By matching the age of each
pulsar, we determine their magnetic field and spin period at birth and confirm
the magnetar-strength field of PSR J1734-3333. Our results indicate that all
three pulsars formed in a similar way to central compact objects (CCOs), with
differences due to the amount of accreted mass. We suggest that magnetic field
emergence may play a role in the distinctive glitch behaviour of low braking
index pulsars, and we propose glitch behaviour and characteristic age as
possible criteria in searches for CCO descendants.
