  The statistical properties of circumstellar disks around young stars are
important for constraining theoretical models for the formation and early
evolution of planetary systems. In this brief review, I survey the literature
related to ground-based and Spitzer-based infrared (IR) studies of young
stellar clusters, with particular emphasis on tracing the evolution of
primordial (``protoplanetary'') disks through spectroscopic and photometric
diagnostics. The available data demonstrate that the fraction of young stars
with optically thick primordial disks and/or those which show spectroscopic
evidence for accretion appears to approximately follow an exponential decay
with characteristic time ~2.5 Myr (half-life = 1.7 Myr). Large IR surveys of
~2-5 Myr-old stellar samples show that there is real cluster-by-cluster scatter
in the observed disk fractions as a function of age. Recent Spitzer surveys
have found convincing evidence that disk evolution varies by stellar mass and
environment (binarity, proximity to massive stars, and cluster density).
Perhaps most significantly for understanding the planeticity of stars, the disk
fraction decay timescale appears to vary by stellar mass, ranging from ~1 Myr
for >1.3 Msun stars to ~3 Myr for <0.08 Msun brown dwarfs. The exponential
decay function may provide a useful empirical formalism for estimating very
rough ages for YSO populations and for modeling the effects of disk-locking on
the angular momentum of young stars.
