Commanders and other units age one year every time Late Winter comes around, i.e. every 12th turn. At a certain age, called the max-age, they gain the Old Age attribute:
(ie. Old Age(65/44) = 21 difference = (Str-3, Att-3, Def-3, Prec-2, Enc +3, HP -15%, CS-15%, MM-6))
They also risk gaining an affliction every late winter.
Recuperation cannot heal afflictions from units suffering from old age.
Some units start with old age and most of the time will be shown in the recruitment screen. This can sometimes be false as it doesn't factor in age modifiers like magic sites or random paths.
A unit's max age is modified by the following (in order of priority):
boosters/empower to remove old age or make recuperation work, accidental F old-age, old age making non-old-age afflictions unhealable