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 \arcdeg , 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 ( \sim 10 % ) we conclude that the 25 Ori group is of comparable age to the 11 Myr Orion OB1a subassociation .