The question: at what age do orcs participate in the om’riggor?
The answer: The Om’riggor, or the orcish rite of adulthood, is mentioned three times in the text of Christie Golden’s Rise of the Horde.
Piecing together all of the specific and relative time references used in the text, we can extrapolate this:
At age six, orcs begin learning how to use weapons.
At age twelve, young orcs can join in the less-dangerous hunts for talbuk.
Orgrim and Durotan became friends at the autumn Kosh’harg. Their friendship continues into the next summer. When they encounter the ogre during their contest in Terokkar Forest, Durotan recalled going on the talbuk hunts, but wasn’t yet of the age to join in the hunts against the ogres or gronn. This implies that there is a non-trivial gap in time between when an orc participates in the talbuk hunts and when he can join the ogre hunts.
Orgrim and Durotan undergo their om’riggor (in summer and autumn respectively) after meeting the draenei.
Orgrim and Durotan’s first Kosh’harg as adults is once again in the autumn.
What’s concrete is that Orgrim and Durotan are both older than twelve when they perform the om’riggor.
What’s not as concrete is whether or not they had participated in the talbuk hunts before they became friends, since that happened in the last year before they participated in the om’riggor.
Conclusion: It is safe to operate under the assumption that an orc who has passed his/her om’riggor is at least twelve. It is probably more feasible that the age for the om’riggor is greater than that, but no other concrete evidence is given to delineate what it could be.