  The similarity of the host galaxy of FRB 121102 with those of long gamma-ray
bursts and Type I super-luminous supernovae suggests that this FRB could be
associated with a young magnetar. By assuming the FRB emission to be produced
within the magnetosphere, we derive a lower limit on the age of the magnetar,
after which GHz emission is able to escape freely from the dense relativistic
wind of the magnetar. Another lower limit is obtained by requiring the
dispersion measure contributed by the electron/positron pair wind to be
consistent with the observations of the host galaxy. Furthermore, we also
derive some upper limits on the magnetar age with discussions on possible
energy sources of the FRB emission and the recently-discovered persistent radio
counterpart. As a result, some constraints on model parameters are addressed by
reconciling the lower limits with the possible upper limits that are derived
with an assumption of rotational energy source.
