Seniority in the marines just means you re-enlisted.
Seniority in the military means you have been around and done quite a
lot of things. For officers if you make O-6 before 18 years in you are
on the fast track. If you haven't made it by 22 years you are just
marking time before you retire.
In the military seniority means you have been exposed to quite a bit of
stuff. My cousin is finally getting a battalion. He has been at NATO for
the last 18 months and he has been in Afghanistan for two tours and Iraq
for two tours. He is well on his way to O-6 and beyond.
Don't forget about enlisted time for us draftees!!