  We report 24 and/or 70 um measurements of ~160 A-type main-sequence stars
using the Multiband Imaging Photometer for Spitzer (MIPS). Their ages range
from 5 to 850 Myr based on estimates from the literature (cluster or moving
group associations) or from the H-R diagram and isochrones. The thermal
infrared excess is identified by comparing the deviation (~3% and ~15% at the
1-\sigma level at 24 and 70 um, respectively) between the measurements and the
synthetic Kurucz photospheric predictions. Stars showing excess infrared
emission due to strong emission lines or extended nebulosity seen at 24 um are
excluded from our sample; therefore, the remaining infrared excesses are likely
to arise from circumstellar debris disks. At the 3-sigma confidence level, the
excess rate at 24 and 70 um is 32% and >=33% (with an uncertainty of 5%),
considerably higher than has been found for old solar analogs and M dwarfs. Our
measurements place constraints on the fractional dust luminosities and
temperatures in the disks. We find that older stars tend to have lower
fractional dust luminosity than younger ones. While the fractional luminosity
from the excess infrared emission follows a general 1/t relationship, the
values at a given stellar age vary by at least two orders of magnitude. We also
find that (1) older stars possess a narrow range of temperature distribution
peaking at colder temperatures, and (2) the disk emission at 70 um persists
longer than that at 24 um. Both results suggest that the debris-disk clearing
process is more effective in the inner regions.
