^ Decker on the other hand seems too old to be a third year academy cadet.
At the time that TMP was released, Stephen Collins had 32 years, Captain Decker could have initially been give captaincy of the Enterprise while still holding the rank Commander. Officers in Starfleet seem to rise through the ranks faster than officers today.
More like the speed we saw during major conflicts like WWII. According to Gene Roddenberry's novelization, losses of starships during that time period were heavy, officers would have come up faster, and received commands earlier.