  [abridged] Protoplanetary disks with AU-scale inner clearings, often referred
to as transitional disks, provide a unique sample for understanding disk
dissipation mechanisms and possible connections to planet formation.
Observations of young stellar clusters with the Spitzer Space Telescope have
amassed mid-infrared spectral energy distributions for thousands of star-disk
systems from which transition disks can be identified. From a sample of 8
relatively nearby young regions (d <= 400 pc), we have identified about 20 such
objects, which we term "classical" transition disks, spanning a wide range of
stellar age and mass. We also identified two additional categories representing
more ambiguous cases: "warm excess" objects with transition-like spectral
energy distributions but moderate excess at 5.8 microns, and "weak excess"
objects with smaller 24 micron excess that may be optically thin or exhibit
advanced dust grain growth and settling. From existing Halpha emission
measurements, we find evidence for different accretion activity among the three
categories, with a majority of the classical and warm excess transition objects
still accreting gas through their inner holes and onto the central stars, while
a smaller fraction of the weak transition objects are accreting at detectable
rates. We find a possible age dependence to the frequency of classical
transition objects, with fractions relative to the total population of disks in
a given region of a few percent at 1-2 Myr rising to 10-20% at 3-10 Myr. The
trend is even stronger if the weak and warm excess objects are included.
Classical transition disks appear to be less common, and weak transition disks
more common, around lower-mass stars (M <= 0.3 Msun).
