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 \geq 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 lumnosity .
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 .