  We analyze samples of Spitzer Infrared Spectrograph (IRS) spectra of T Tauri
stars in the Ophiuchus, Taurus, and Chamaeleon I star-forming regions, whose
median ages lie in the <1 to 2 Myr range. The median mid-infrared spectra of
objects in these three regions are similar in shape, suggesting, on average,
similar disk structures. When normalized to the same stellar luminosity, the
medians follow each other closely, implying comparable mid-infrared excess
emission from the circumstellar disks. We use the spectral index between 13 and
31 micron and the equivalent width of the 10 micron silicate emission feature
to identify objects whose disk configuration departs from that of a continuous,
optically thick accretion disk. Transitional disks, whose steep 13-31 micron
spectral slope and near-IR flux deficit reveal inner disk clearing, occur with
about the same frequency of a few percent in all three regions. Objects with
unusually large 10 micron equivalent widths are more common (20-30%); they
could reveal the presence of disk gaps filled with optically thin dust. Based
on their medians and fraction of evolved disks, T Tauri stars in Taurus and
Chamaeleon I are very alike. Disk evolution sets in early, since already the
youngest region, the Ophiuchus core (L1688), has more settled disks with larger
grains. Our results indicate that protoplanetary disks show clear signs of dust
evolution at an age of a few Myr, even as early as ~1 Myr, but age is not the
only factor determining the degree of evolution during the first few million
years of a disk's lifetime.
