Age-dating of the oldest known objects in the Universe has been carried out in a number of ways. The most reliable ages are generally believed to come from the application of theoretical models of stellar evolution to observations of the oldest clusters in the Milky Way, the globular clusters. For about 30 years, the ages of globular clusters have remained reasonably stable, at about 15 billion years (e.g., Demarque et al. 1991; Vandenberg et al. 1996); however, recently these ages have been revised downward, as described below. Ages can also be obtained from radioactive dating or nucleocosmochronology (e.g., Schramm 1989), and a further lower limit can be estimated from the cooling rates for white dwarfs (e.g., Oswalt et al. 1996). Generally, these ages have ranged from about 10 to 20 billion years; the largest sources of uncertainty in each of these estimates are again systematic in nature.
During the 1980's and 1990's, the globular cluster age estimates have improved as both new observations of globular clusters have been made with CCD's, and as refinements to stellar evolution models, including updated opacities, consideration of mixing, and different chemical abundances, have been incorporated (e.g., Vandenberg et al. 1996; Chaboyer et al. 1996, 1998). The latter authors have undertaken an extensive analysis of the uncertainties in the determination of globular cluster ages. From the theory side, uncertainties in globular cluster ages result, for example, from uncertainties in how convection is treated, the opacities, and nuclear reaction rates. From the measurement side uncertainties arise due to corrections for dust and chemical composition; however, the dominant source of overall uncertainty in the globular cluster ages is the uncertainty in the cluster distances.
In fact, the impact of distance uncertainty on the ages of globular clusters is twice as large as its effect on the determination of H0. That is, a 0.2 mag difference in zero point corresponds to a 10% difference in the distance (or correspondingly, H0), but it results in a 20% difference in the age of a cluster (e.g., Renzini 1991).
The Hipparcos satellite has recently provided geometric parallax measurements for 118,000 nearby stars (Kovalevsky 1998). Relevant for the calibration of globular cluster distances, are the relatively nearby old stars of low metal composition, the so-called subdwarf stars, presumably the nearby analogs of the old, metal-poor stars in globular clusters. Accurate distances to these stars provide a fiducial calibration from which the absolute luminosities of equivalent stars in globular clusters can be determined and compared with those from stellar evolution models. The new Hipparcos calibration has led to a downward revision of the globular cluster ages from ~ 15 billion years to 11-14 billion years (e.g., Reid 1997; Pont et al. 1998; Chaboyer et al. 1998).
However, as emphasized by Chaboyer et al., there are only 8 stars in the Hipparcos catalog having both small parallax errors, and low metal abundance, [Fe/H] < -1.1 (i.e., less than one tenth the iron-to-hydrogen abundance relative to the solar value). In fact, Gratton et al. (1998) note that there are no stars with parallax errors < 10% with [Fe/H] < -2 corresponding to the oldest, metal poor globular clusters. Hence, the calibration of globular cluster ages based on parallax measurements of old, metal-poor stars remains an area where an increase in sample size will be critical to beat down the statistical uncertainties. A decade or so from now, new parallax satellites such as NASA's SIM (the Space Interferometry Mission) and the European Space Agency's mission (named GAIA) will be instrumental in improving these calibrations, not only for subdwarfs, but for many other classes of stars (for example, Cepheids and the lower-mass variable RR Lyrae stars). These interferometers will be capable of delivering 2-3 orders of magnitude more accurate parallaxes than Hipparcos, down to fainter magnitude limits for several orders of magnitude more stars. Until larger samples of accurate parallaxes are available, however, distance errors are likely to continue to contribute the largest source of systematic uncertainty to the globular cluster ages.