  We investigate the agegraphic dark energy models which were recently proposed
to explain the dark energy-dominated universe. For this purpose, we calculate
their equation of states and squared speeds of sound. We find that the squared
speed for agegraphic dark energy is always negative. This means that the
perfect fluid for agegraphic dark energy is classically unstable. Furthermore,
it is shown that the new agegraphic dark energy model could describe the matter
(radiation)-dominated universe in the far past only when the parameter $n$ is
chosen to be $n>n_c$, where the critical values are determined to be
$n_c=2.6878(2.5137752)$ numerically. It seems that the new agegraphic dark
energy model is no better than the holographic dark energy model for the
description of the dark energy-dominated universe, even though it resolves the
causality problem.
