Radiator/Coolant Flush - How often?

What is your question?

On an older car, are you suppose to have the coolant flushed more that once every 2 years? I went to Duke of Oil for an oil change and they said the coolant service was necessary as the engine could crack or cracks could happen in other areas. I told them it had been done just a year ago, but, they insisted it was needed. Did I just get scammed or because the car is 16 years old it was necessary? They showed me a tube of the coolant and it didn't look murky or cloggy looking.