  We present a Spitzer MIPS study of the decay of debris disk excesses at 24
and 70 $\mu$m for 255 stars of types F4 - K2. We have used multiple tests,
including consistency between chromospheric and X-ray activity and placement on
the HR diagram, to assign accurate stellar ages. Within this spectral type
range, at 24 $\mu$m, $13.6 \pm 2.8 \%$ of the stars younger than 5 Gyr have
excesses at the 3$\sigma$ level or more, while none of the older stars do,
confirming previous work. At 70 $\mu$m, $22.5 \pm 3.6\%$ of the younger stars
have excesses at $ \ge$ 3 $\sigma$ significance, while only
$4.7^{+3.7}_{-2.2}$% of the older stars do. To characterize the far infrared
behavior of debris disks more robustly, we double the sample by including stars
from the DEBRIS and DUNES surveys. For the F4 - K4 stars in this combined
sample, there is only a weak (statistically not significant) trend in the
incidence of far infrared excess with spectral type (detected fractions of
21.9$^{+4.8}_{-4.3}\%$, late F; 16.5$^{+3.9}_{-3.3}\%$, G; and
16.9$^{+6.3}_{-5.0}\%$, early K). Taking this spectral type range together,
there is a significant decline between 3 and 4.5 Gyr in the incidence of
excesses with fractional luminosities just under $10^{-5}$. There is an
indication that the timescale for decay of infrared excesses varies roughly
inversely with the fractional luminosity. This behavior is consistent with
theoretical expectations for passive evolution. However, more excesses are
detected around the oldest stars than is expected from passive evolution,
suggesting that there is late-phase dynamical activity around these stars.
