[Taqwus]

All units and commanders, even immortal ones, have both an age value and an old-age threshold. The usual rate of aging is one month per game turn, although Burden of Time increases this dramatically, and units affected by Decay age five years per combat turn, and there's a couple of magic items which eliminate aging for their owner.

The thresholds depend on the unit type and, for commanders, their magic paths; ordinary humans tend to have a 50-year threshold for old age, but fire magic decreases this while nature magic increases it. A Treelord, on the other hand, might have a threshold in the scale of thousands of years, and thus a non-old Treelord is highly unlikely to actually become old age during a game unless he's spending all his time being hacked by people with bane blades.