  We report the results of the first sensitive L-band (3.5 micron) survey of
the intermediate age (2.5 - 30 Myr) clusters NGC 2264, NGC 2362 and NGC 1960.
We use JHKL colors to obtain a census of the circumstellar disk fractions in
each cluster. We find disk fractions of 52% +/- 10%, 12% +/- 4% and 3% +/- 3%
for the three clusters respectively. Together with our previously published
JHKL investigations of the younger NGC 2024, Trapezium and IC 348 clusters, we
have completed the first systematic and homogenous survey for circumstellar
disks in a sample of young clusters that both span a significant range in age
(0.3 - 30 Myr) and contain statistically significant numbers of stars whose
masses span nearly the entire stellar mass spectrum. Analysis of the combined
survey indicates that the cluster disk fraction is initially very high (> 80%)
and rapidly decreases with increasing cluster age, such that half the stars
within the clusters lose their disks in < ~3 Myr. Moreover, these observations
yield an overall disk lifetime of ~ 6 Myr in the surveyed cluster sample. This
is the timescale for essentially all the stars in a cluster to lose their
disks. This should set a meaningful constraint for the planet building
timescale in stellar clusters. The implications of these results for current
theories of planet formation are briefly discussed.
