  We study the evolution of circumstellar disks in 22 young (1 to 100 Myr)
nearby (within 500 pc) associations over the entire mass spectrum using
photometry covering from the optical to the mid-infrared. We compiled a catalog
of 2340 spectroscopically-confirmed members of these nearby associations. We
analyzed their spectral energy distributions and searched for excess related to
the presence of protoplanetary disks in a homogeneous way. Sensitivity limits
and spatial completeness were also considered. We derive disk fractions as
probed by mid-infrared excess in these regions. The unprecedented size of our
sample allows us to confirm the timescale of disk decay reported in the
literature and to find new trends. The fraction of excess sources increases
systematically if measured at longer wavelengths. Disk percentages derived
using different wavelength ranges should therefore be compared with caution.
The dust probed at 22-24 um evolves slower than that probed at shorter
wavelengths (3.4-12 um). Assuming an exponential decay, we derive a timescale
tau=4.2-5.8 Myr at 22-24 um for primordial disks, compared to 2-3 Myr at
shorter wavelength (3.4-12 um). Primordial disks disappear around 10 Myr,
matching in time a brief increase of the number of 'evolved' disks. The
increase in timescale of excess decay at longer wavelength is compatible with
inside-out disk clearing scenarios. The increased timescale of decay and larger
dispersion in the distribution of disk fractions at 22-24 um suggest that the
inner and outer zones evolve differently, the latter potentially following a
variety of evolutionary paths. The drop of primordial disks and the coincident
rise of evolved disks at 10 Myr are compatible with planet formation theories
suggesting that the disappearance of the gas is immediately followed by the
dynamical stirring of the disk.
