Final answer:
Hyponatremia is the age-related change that requires careful monitoring of fluid and electrolyte balance in older adults.
Step-by-step explanation:
The age-related change that causes the nurse to carefully monitor the client's fluid and electrolyte balance is hyponatremia. Hyponatremia refers to lower-than-normal levels of sodium in the blood. This imbalance can result in increased entry of water into cells, causing swelling and potential damage to red blood cells and neurons. Therefore, it is important for the nurse to monitor the client's fluid and electrolyte balance to prevent complications.