The HST/ACS GGC Survey provides reliable photometry from 6 magnitudes below the main sequence turn-off to the level of the horizontal branch ( 3.5 magnitudes brighter than the turn-off) for 65 Galactic globular clusters. The absolute ages of a sub-sample of those clusters, which have low reddenings and well determined chemical compositions, have been determined. The ages were derived from a large Monte Carlo simulation, in which the various physical inputs required to produce isochrones (such as nuclear reaction rates, opacities and colour transformations) were varied within their known uncertainties. Each Monte Carlo isochrone was tested to see if it provided a reasonable fit to the colours and absolute magnitudes of local calibrating sub-dwarfs. Isochrones that were unable to correctly reproduce the position of the local calibrating stars in a colour-magnitude diagram were discarded. Isochrones passing this test were used to determine the distance to the cluster, via main sequence fitting between the theoretical isochrone and the observed main sequence in each of the individual globular clusters. The age of the globular cluster was then determined from the absolute magnitude of the point on the sub-giant branch that is 0.04 mag redder than the turn-off. This simulation takes into account all sources of error in the age determination process and leads to a robust determination of the absolute ages of the globular clusters. Support for this work (GO-10775) was provided by NASA through a grant from the Space Telescope Science Institute which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555.