Age is a form of Natural Signamancy. Erfworlders do not die directly of old age, and a unit that is crucial to their side can maintain their youthful Signamancy indefinitely. But when their talents are inevitably wasted, or if they put a serious hardship on their side, it will put a strain on them that manifests as old age and other detrimental Signamancy. When a typical unit grows too weak, their ruler will usually disband them. When a ruler becomes too weak, they will become feeble minded, leaving their side open to conquest. The oldest any unit is believed to be is 100,000 turns, or 273.9 years.
It is possible for a unit that has "aged" to be restored to youthful Signamancy. Maggie, for example, was left severely weakened by a long term Eyemancy caster link. But as she gets more involved in her side, and pursues her implied crush on Parson, her Signamancy has greatly improved.