All orcs of an age set are initiated into orchood at the same time, and after the secret ceremony they are sent off naked and unarmed on the backs of their dogs into the lowlands, not to come back until they have survived for one moon on their own. Some do not make it back, but those who do have forged an unbreakable connection with each other. Each age set acts as a unit in combat, and it is this organization that makes the hill orcs so effective on the battlefield even though they are not tirelessly drilled and regimented like the grey orcs.
Here is a sheet I used for a hill orc encounter in the orcish lowlands - I statted the orcs up as barbarians (using my super light version of 3.5) and sketched a quick drawing illustrating the characteristic floppy brimmed hat, kilt, polearm and shield that all hill orcs carry. These orcs are travelling on foot, which is a rarity.
Thanks Eli Arndt for the awesome drawing of a hill orc!