• Age the unit 5 years per combat round. The number is modified by the unit's current aging rate (unaging bless, quickness, item modifiers, etc etc). This in itself can cause damage and afflictions, as below.
• Remember how many years this actually added (after modifiers). If this was below 5, treat it as 5. This value is used to make the white number on the battle replay.
• You take additional damage. This amount is calculated as floor((old age penalty * 5 + [random number 0-9] + number of years actually added in the first step)/10).
• There is a 1% chance that affected units are allowed to make a check to stop decaying of 15 + DRN vs MR + DRN. Unlike regular MR checks, a tie on the defender's side counts as a success.

This applies to all forced age changes as well as the natural one applied every midwinter.

If the number of years is negative, simply subtract that from the unit's current age, and don't do anything else.

If your aging rate is greater than 100 (this is measured in years per century), the number of years becomes:

 (Age rate * number of years to add + [a random number 0-99]) / 100

With that out the way, do this once for each year to add to the unit:

• If your age rate is less than 100, pick a random number between 0 and 99. If this >= your age rate, you skip aging and all its effects this time.
• If your age < your max age, increment your age by 1 and suffer no other adverse effects.
• Calculate some value: 500 / max age, capped to a maximum value of 10.
• If you are on the map (province ID is positive, so not in another plane), the value becomes ((death scale + 10) * (old value) * 10)/100.
• If a random number 0-999 is less than (value * 15), you get to ride the affliction train.
  • 35% chance of disease. Cannot happen if already diseased.
  • Otherwise, there is a 75% chance that you get a random minor affliction. If this doesn't happen you get a random major affliction instead.
  • If something above gave you battle fright, remove it.
  • If something above gave you disease, you have a 50% chance per death path (rolled independently) to have your disease removed.
• If you are 2x your max age (or older), you take 1 point of damage (no rolls).
• If you are diseased and over your maxage, you take 1 point of damage (no rolls).

• 10% for undocumented affliction 16. I don't know what this actually does but there seems to be some very unique messages for certain chassis. If you get this, exit.
• If you have the miscellaneous body type:
  • 90% to skip the rest of this section.
  • 50% to enter this section:
    • If you are mindless or a slave vessel, leave without any ill effects.
    • 50% to gain undocumented affliction 137438953472 and leave, but only if you do not have it already.
  • Otherwise:
    • If you have more than one arm, lose an arm and exit.
    • If you are not mindless and not inanimate, lose an eye if you have one. Either way, exit.
    • Otherwise, gain undocumented affliction 2147483648 and become mute. Exit the affliction train only if mindless or a slave vessel.
  • Upon making it to this point, become feebleminded and exit.
• If you do not have the miscellaneous body type:
  • ???

• 10% for undocumented affliction 16. I don't know what this actually does but there seems to be some very unique messages for certain chassis. If you get this, exit.
• If in battle: 10% chance if not ethereal, undead or inanimate (and not already bleeding) to start bleeding and exit.
• 20% chance to get battle fright if your base morale is less than 25. Even if your morale is 25+ this is rolled. Exit after applying battle fright (or not).
• More of the bizarre bodyshape stuff that I am clearly not understanding properly