  We examine the age-dependence of dark matter halo clustering in an
unprecedented accuracy using a set of 7 high-resolution cosmological
simulations each with $N=1024^3$ particles. We measure the bias parameters for
halos over a large mass range using the cross-power-spectrum method that can
effectively suppress the random noise even in the sparse sampling of the most
massive halos. This enables us to find, for the first time, that younger halos
are more strongly clustered than older ones for halo masses $M>10M_{\ast}$,
where $M_\ast$ is the characteristic nonlinear mass scale. For $M<M_{\ast}$,
our results confirm the previous finding of Gao et al. that older halos are
clustered more strongly than the younger ones. We also study the halo bias as a
function of halo concentration, and find that the concentration dependence is
weaker than the age dependence for $M<M_{\ast}$, but stronger for $M\ga 50
M_{\ast}$. The accurate and robust measurement of the age dependences of halo
bias points to a limitation of the simple excursion set theory which predicts
that the formation and structure of a halo of given mass is independent of its
environment.
