  The energy densities of matter and the vacuum are currently observed to be of
the same order of magnitude: $(\Omega_{m 0} \approx 0.3) \sim (\Omega_{\Lambda
0} \approx 0.7)$. The cosmological window of time during which this occurs is
relatively narrow. Thus, we are presented with the cosmological coincidence
problem: Why, just now, do these energy densities happen to be of the same
order? Here we show that this apparent coincidence can be explained as a
temporal selection effect produced by the age distribution of terrestrial
planets in the Universe. We find a large ($\sim 68 %$) probability that
observations made from terrestrial planets will result in finding $\Omega_m$ at
least as close to $\Omega_{\Lambda}$ as we observe today. Hence, we, and any
observers in the Universe who have evolved on terrestrial planets, should not
be surprised to find $\Omega_m \sim \Omega_{\Lambda}$. This result is
relatively robust if the time it takes an observer to evolve on a terrestrial
planet is less than $\sim 10$ Gyr.
