I think his point is that a Hastati unit in a game such as TW is not the same group of people fighting over a number of years, such that after a certain number of years they all progress to the rank of Principes because they have all reached that age. A unit in TW is just a body of men of a certain class, the individuals that make up that class will certainly change over time. Over time the individuals who make up that unit will change in small numbers. Perhaps in one week, a group of 5 or so reach an age where they are then reorganised into a Principes unit, while in that same week five new individuals replace them from some other unit.
You are trying to represent minutiae as an important gameplay mechanic, and so it stops being historical and starts being gamey. Its important to maintain the correct perspective across the game to ensure that what you represent is relevant to your representation. Focusing on small things that aren't important to your chosen perspective is what TW do, and what I wish they wouldn't.
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