The average lifespan of a pond-raised goldfish is 12 years. Your have lived beyond this date so you did nothing wrong.
Unfortunately fish, very much like all organisms, become far more susceptible to sickness as they age, so if those goldfish were all 15 year old and perhaps genetically closely related there's no need to beat yourself (or the terrapin) over it. They lived longer than average.
Get some new goldfish... assuming you haven't got herons. They are by far the first cause of death for pond fish around here.
