Time Variable Cosmological Constants from the Age of Universe
  In this paper, time variable cosmological constant, dubbed {\it age
cosmological constant}, is investigated motivated by the fact: any cosmological
length scale and time scale can introduce a cosmological constant or vacuum
energy density into Einstein's theory. The age cosmological constant takes the
form $\rho_{\Lambda}=3c^2M^2_P/t_{\Lambda}^2$, where $t_{\Lambda}$ is the age
of our universe or conformal time. The effective equation of state of age
cosmological constant are
$w^{eff}_{\Lambda}=-1+{2/3}\frac{\sqrt{\Omega_{\Lambda}}}{c}$ and
$w^{eff}_{\Lambda}=-1+{2/3}\frac{\sqrt{\Omega_{\Lambda}}}{c}(1+z)$ when the age
of universe and conformal time are taken as the role of cosmological time
scales respectively. They are the same as the so-called agegraphic dark energy
models. However, the evolution history are different from the agegraphic ones
for their different evolution equations.
