"Date Of Hire AND Birthday, with oldest on top."
Utter twaddle!!! You can be 65 with 200 hours.
Seniority implies date of hire only.
When designating command other factors come into play. Licence is obvious (ATP / Comm). Are you eligible for command?
Then it becomes a matter of company preference. The best one I have worked with is the rule of 3 :
A. Seniority
B. Total Time
C. Time on Type
(in any order)
Three factors ensure a "winner". Each category scores a point for the highest value and the one with at least 2/3 is designated.
Finish! Easy! Acceptable!