How to tell
Overwatering & rot at the base. The soil stays wet for days, and the rot starts low: a soft, discolored patch at the base or crown rather than one particular old leaf, often paired with yellowing stalks.
Too little light. The yellowing comes with pale, stretched new growth leaning toward the nearest window. That combination shows that the spot is too dim for ZZ plant.
Thirst (underwatering). The soil is bone dry, and the yellowing arrives alongside wrinkled stalks rather than soft, mushy growth.
Hungry (but rarely the real culprit). ZZ plant grows slowly and needs very little feeding, so a uniform pale-yellow fade after many months without any is possible, but check watering and light first: those are far more common on this plant.
Simple old age. An older, outer leaf near the base slowly yellows on its own while the center keeps producing healthy new growth (normal turnover on a ZZ plant, not a symptom).
