Robbie started as a "Marine" as an enlisted man, served enough time to be promoted to E3 when he saved the son of General Thompson. Due to the MOS being fairly stable in an extended time of peace, advancement was limited to attrition by retirement. Robbie would never have considered a commissioned position due to his self-worth issues and found himself "forced" into a position in the Academy as a reward at an age that was far beyond the norm of the other candidates in the Academy at the time. He was still having difficulty being "senior" in rank to older marines. Then again, there is the fact that this takes place in a time when interstellar travel is possible so the Corps has adapted a bit to changing times, like extended average lifespans so spending 2 years instead of the 6 months a PFC could expect before a promotion was not unusual.