A unit that is decaying is aged 5 years per combat round and has a variable chance to take damage based on the age of the unit. The number of years added is modified by normal effects that affect age rate, such as quickness, slow, or an unaging bless. Affected units may then take damage, which is calculated as (Old Age penalty * 5 + closed d10 - 1 + max(5 number of years actually aged))/10, rounded down. Additionally they are subject to the usual effects of age, which may include gaining afflictions and suffering additional damage if the unit is old.
The text displayed in white in battle replays on decaying units will read "5" even if the actual number of years the unit aged is less than that. If it is larger, the text will be correct however.
Like poison and profuse bleeding, decay will continue to apply its effects to units in the 20 turns following completed combat. A unit that retreats off the field suffers 100 simulated rounds of various afflictions, including decay.