There is no magic mileage when a car suddenly becomes too old to repair, even when the odometer rolls past 150,000 or 200,000 miles. A well-maintained car with a healthy engine, solid transmission, and clean body can make more financial sense to repair than replacing it with another used vehicle that comes with its own mystery problems.
That makes the mileage number useful, but not decisive. The better question asks what the car needs now, what it will likely need next, and how much reliable driving that repair could buy. A $2,500 repair can look painful on a Tuesday afternoon, but it may look downright reasonable beside years of payments, insurance costs, registration fees, and the unknown condition of another vehicle.