For day to day decisions, the better question is not whether the unit can be repaired but whether the cost of the repair relative to the unit's remaining useful life makes sense. A tank water heater in its second decade with a failing element and a depleted anode rod has a different calculus than a six-year-old unit with a straightforward thermostat fault.
On the ground, the evidence points to age and condition as the two deciding factors. A unit under ten years old with a single identifiable fault is usually worth repairing. One that is older than twelve years with multiple symptoms is typically closer to replacement territory, especially if the tank has not been flushed regularly.
Next Steps: Start with the most visible symptom — inconsistent temperature, no hot water, unusual sounds, or visible corrosion around the base — and work from there. Each symptom points toward a specific component. A plumber who knows the local water conditions can inspect the anode rod and tank interior and give a direct read on whether repair or replacement is the better path for the specific unit and water supply.