Dark Energy and the Hubble Age

I point out that an effective upper limit of approximately 20 Gyr (for a Hubble constant of 72 km/s/Mpc) or alternatively on the $H_0$-independent quantity $H_0t_0 < 1.47$, exists on the age of the Universe, essentially independent of the unknown equation of state of the dominant dark energy component in the Universe. Unless astrophysical constraints on the age of the Universe can convincingly reduce the upper limit to below this value no useful lower limit on the equation of state parameter $w$ for this component can be obtained. Direct dating by stars does not provide a useful constraint, but model-dependent cosmological limits from supernovae and the CMB observations may. For a constant value of $w$, a bound $H_0t_0 < 1.1$ gives a limit $w> -1.5$