  We present recent results from our ongoing large scale variability survey of
the Orion OB1 Association. In an area of ~ 25 square degrees we have unveiled
new populations of low-mass young stars over a range of environments, and ages
from 1-2 Myr in Ori OB 1b to roughly 10 Myr in Ori OB 1a. There is a lack of
strong Ha emission and near-IR excesses in the young stars of Ori OB 1a,
suggesting that accretion stops and disks dissipate for most solar type stars
in a few Myr, probably caused by of the onset of planet formation. The absence
of dust and gas in Ori OB 1a is consistent with the picture of star formation
as a rapid process, in which molecular clouds dissipate in just a few Myr after
the first stars are born.
