  We present Spitzer 3.6--24 micron photometry and spectroscopy for stars in
the 1--3 Myr-old Coronet Cluster, expanding upon the survey of Sicilia-Aguilar
et al. (2008). Using sophisticated radiative transfer models, we analyze these
new data and those from Sicilia-Aguilar et al. (2008) to identify disks with
evidence for substantial dust evolution consistent with disk clearing:
transitional disks. We then analyze data in Taurus and others young clusters --
IC 348, NGC 2362, and eta Cha -- to constrain the transitional disk frequency
as a function of time. Our analysis confirms previous results finding evidence
for two types of transitional disks -- those with inner holes and those that
are homologously depleted. The percentage of disks in the transitional phase
increases from ~ 15--20% at 1--2 Myr to > 50% at 5--8 Myr; the mean
transitional disk lifetime is closer to ~ 1 Myr than 0.1--0.5 Myr, consistent
with previous studies by Currie et al. (2009) and Sicilia-Aguilar et al.
(2009). In the Coronet Cluster and IC 348, transitional disks are more numerous
for very low-mass M3--M6 stars than for more massive K5--M2 stars, while Taurus
lacks a strong spectral type-dependent frequency. Assuming standard values for
the gas-to-dust ratio and other disk properties, the lower limit for the masses
of optically-thick primordial disks is Mdisk ~ 0.001--0.003 M*. We find that
single color-color diagrams do not by themselves uniquely identify transitional
disks or primordial disks. Full SED modeling is required to accurately assess
disk evolution for individual sources and inform statistical estimates of the
transitional disk population in large samples using mid-IR colors.
