  Motivated by recent developments in particle physics and cosmology, there has
been growing interest in an unified description of dark matter and dark energy
scenarios. In this paper we explore observational constraints from age
estimates of high-$z$ objects on cosmological models dominated by an exotic
fluid with equation of state $p = -A/\rho^{\alpha}$ (the so-called generalized
Chaplygin gas) which has the interesting feature of interpolating between
non-relativistic matter and negative-pressure dark energy regimes. As a general
result we find that, if the age estimates of these objects are correct, they
impose very restrictive limits on some of these scenarios.
