We review five independent techniques which are used to set the distance scale to globular clusters , including subdwarf main sequence fitting utilizing the recent Hipparcos parallax catalogue .
These data together all indicate that globular clusters are farther away than previously believed , implying a reduction in age estimates .
We now adopt a best fit value \hbox { $ M _ { v } ( RR ) $ } = 0.39 \pm 0.08 ( stat ) at \hbox { $ [ { Fe } / { H } ] $ } = -1.9 with an additional uniform systematic uncertainty of ^ { +0.13 } _ { -0.18 } .
This new distance scale estimate is combined with a detailed numerical Monte Carlo study ( previously reported by Chaboyer et al . 1996a ) designed to assess the uncertainty associated with the theoretical age-turnoff luminosity relationship in order to estimate both the absolute age and uncertainty in age of the oldest globular clusters .

Our best estimate for the mean age of the oldest globular clusters is now 11.5 \pm 1.3 Gyr , with a one-sided , 95 % confidence level lower limit of 9.5 Gyr .
This represents a systematic shift of over 2 \sigma compared to our earlier estimate , due completely to the new distance scale—which we emphasize is not just due to the Hipparcos data .
This now provides a lower limit on the age of the universe which is consistent with either an open universe , or a flat , matter dominated universe ( the latter requiring H _ { 0 } \leq 67 \hbox { $ { km } { s } ^ { -1 } { Mpc } ^ { -1 } $ } ) .
Our new study also explicitly quantifies how remaining uncertainties in the distance scale and stellar evolution models translate into uncertainties in the derived globular cluster ages .
Simple formulae are provided which can be used to update our age estimate as improved determinations for various quantities become available .
Formulae are also provided which can be used to derive the age and its uncertainty for a globular cluster , given the absolute magnitude of the turn-off , or the point on the subgiant branch 0.05 mag redder than the turn-off .