  We present 24 micron Spitzer/MIPS photometric observations of the ~50 Myr
open cluster IC 2391. Thirty-four cluster members ranging in spectral type from
B3-M5 were observed in the central square degree of the cluster. Excesses
indicative of debris disks were discovered around 1 A star, 6 FGK stars, and
possibly 1 M dwarf. For the cluster members observed to their photospheric
limit, we find a debris disk frequency of 10 (-3,+17)% for B-A stars and 31
(-9,+13)% for FGK stars using a 15% relative excess threshold. Relative to a
model of decaying excess frequency, the frequency of debris disks around A-type
stars appears marginally low for the cluster's age while that of FGK stars
appears consistent. Scenarios that may qualitatively explain this result are
examined. We conclude that planetesimal activity in the terrestrial region of
FGK stars is common in the first ~50 Myr and decays on timescales of ~100 Myr.
Despite luminosity differences, debris disk evolution does not appear to depend
strongly on stellar mass.
