  The main sequence turnoff luminosity is the best stellar `clock' which can be
used to determine the absolute ages of globular clusters. This is due to the
fact that it is generally assumed that the luminosity and lifetimes of main
sequence globular cluster stars are independent of the properties of stellar
convection and atmospheres, two areas of stellar evolution which are poorly
understood. Several possible sources of error in this stellar clock are
discussed, and isochrones are constructed using a variety of different physical
assumptions. The mean age of the oldest globular clusters are determined from
these isochrones and it is found that the uncertainties in the input physics
can lead to changes in the derived age of $\pm 15\%$. Surprisingly the largest
source of error is the mixing length theory of convection. It is well known
that uncertainties in the distance scale and chemical composition of globular
cluster stars lead to changes of order $\sim 22\%$ in the determination of
absolute ages. Combining the various sources of error, the absolute age of the
oldest globular clusters are found to lie in the range 11 --- 21 Gyr. This is
meant to be a total theoretical range. For the standard inflationary model
($\Omega = 1, \Lambda = 0$), a minimum age of the universe of 11 Gyr requires
$H_o \la 60~\kmsmpc$.
