  Isochrone computations and horizontal branch (HB) models for Y(MS)=0.23 and
two values of Z (0.0001, 0.0004)are used to derive constraints on the age
indicators and HB morphology of metal-poor clusters with age t (in Gyrs) in the
range of 1< t < 20.It is found that for fixed metallicity the luminosity
L(3.83) of the zero age horizontalbranch (ZAHB) at the RR Lyrae gap depends on
age and that the difference in visual magnitude between main sequence turn-off
(TO) and ZAHB may be calibrated in terms of age and metallicity. The results of
synthetic HB computations show that at fixed metallicity the mean luminosity of
RR Lyraes depends on HB morphology, as well as that the mean luminosity of
purely red horizontal branches depends on the age and amount of mass loss
during the red giant phase.
  Such features should be properly taken into account when the cluster age is
derived from the observed difference in magnitude between main sequence TO and
actual horizontal branch (rather than ZAHB). Moreover, it is shown that the
observed HB morphology provides us with an easy estimate of the upper limit of
the cluster age. Some useful constraints to the cluster metallicity could be
also derived.

Astrophysics