  We report new Spitzer observations of intermediate mass stars in two regions
of the Orion OB1 association located in the subassociations OB1a ($\sim$10 Myr)
and OB1b ($\sim$5 Myr). In a representative sample of stars earlier than F5 of
both stellar groups, we find a population of stars surrounded of debris disks,
without excess in the IRAC bands and without emission lines in their optical
spectra, but with a varying degree of 24{\micron} excess. Comparing our samples
with 24{\micron} observations of intermediate mass stars in other stellar
groups, spanning a range of ages from 2.5 Myr to 150 Myr, we find that debris
disks are more frequent and have larger 24{\micron} excess at 10 Myr (OB1a).
This trend agrees with predictions of models of evolution of solids in the
outer regions of disks ($>$30 AU), where large icy objects ($\sim$1000 Km)
begin to form at $\sim$10 Myr; the presence of these objects in the disk
initiates a collisional cascade, producing enough dust particles to explain the
relatively large 24 {\micron} excess observed in OB1a. The dust luminosity
observed in the stellar groups older than 10 Myr declines roughly as predicted
by collisional cascade models. Combining Spitzer observations, optical spectra
and 2MASS data, we found a new Herbig Ae/Be star (HD290543) and a star
(HD36444) with a large 24 {\micron} excess, both in OB1b. This last object
could be explained as a intermediate stage between HAeBe and true debris
systems or as a massive debris disk produced by a collision between two large
objects ($>$1000 Km).
