You don't need to keep track of every single unit, but say you remember that you recruited that army that's now fighting in Iberia, back in 245 and the year is now 235, you get a new army ready to relieve it so that it can retire soon, it doesn't matter that a few units, are younger or older, it's just a general guideline to make the game more fun.
Just do whatever feels right. 10 years feels right to me because after that long men would be getting older and slowing down, and the life of a soldier would be hard on the men.
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