  The stellar group surrounding the Be (B1Vpe) star 25 Orionis was discovered
to be a pre-main-sequence population by the Centro de Investigaciones de
Astronomia (CIDA) Orion Variability Survey and subsequent spectroscopy. We
analyze Sloan Digital Sky Survey multi-epoch photometry to map the southern
extent of the 25 Ori group and to characterize its pre-main-sequence
population. We compare this group to the neighboring Orion OB1a and OB1b
subassociations and to active star formation sites (NGC 2068/NGC 2071) within
the Lynds 1630 dark cloud. We find that the 25 Ori group has a radius of 1.4
degrees, corresponding to 8-11 pc at the distances of Orion OB1a and OB1b.
Given that the characteristic sizes of young open clusters are a few pc or less
this suggests that 25 Ori is an unbound association rather than an open
cluster. Due to its PMS population having a low Classical T Tauri fraction
(~10%) we conclude that the 25 Ori group is of comparable age to the 11 Myr
Orion OB1a subassociation.
