There is no single age or repair price that answers the question.
A repair can make sense when the system is otherwise reliable and the failure is isolated. Replacement becomes more reasonable when breakdowns are frequent, comfort is poor, efficiency is declining or a major repair is being considered on older equipment.
Compare the repair to the remaining value of the system.
Ask what failed, whether the repair addresses the root problem, whether other major components are showing wear, and what the realistic life expectancy is after the repair.
Comfort problems matter too.
A system that technically runs but cannot control humidity, airflow or room-to-room comfort may need a broader evaluation before more money is put into repairs.
A second opinion is reasonable before a major decision.
If replacement was recommended quickly or the explanation was unclear, another diagnosis can help you compare the findings before committing.