Many of the captains at my Major US carrier where my FO in another life. Some I checked out as captain...in another life.
Seven airlines under my belt with 20 thousand plus hours (mostly PIC) CO-PILOT.
I still get a kick out of captains trying to teach me to fly.
Seniority...it seems unfair to have a more experienced PILOT as SIC.
We in the airline business accept it but it burns me when the average bear thinks the FO is a rookie.