Cohort commander?

[Frank]

In the legions, however, the senior centurions of each cohort seem to have had day-to-day charge. However, there are plenty of examples of vexillations of cohort pairs lead by Tribunes to suggest that the 5 junior tribunes of a legion may well have had defacto authority over a pair of cohorts.

Sure, not all of them were 17 year old trainees. In republican times some Tribuni Laticlavii did more than one campaign, because the minimum age for Quaestor was far away before it was reduced by Augustus. Same for the equestrian Tribuni Angusticlavii who simply needed x campaigns because they were conscriptable as anybody else pre-Marian reforms.

In the empire you have Tribuni which have been promoted from praefectus cohortis. Of course these guys could do that job after 3 years of experience. What scares me more is, that people without any military experience at all did start as praefectus cohortis, if just of equestrian rank.

However, there is no evidence, that there was a fixed organizational structure with 5 tribunes leading 2x5=10 cohors. It seems that the romans have been less eager than we about clear organizational structures.