Goldfish can live for over 40 years, with the oldest recorded in the Guinness Book of Records being an amazing 43 years old.
Determining how old they are is very difficult, particularly with goldfish that are kept in an indoor aquarium. Size is no real guide as this is impacted by the genetics of the fish and the amount of water it is given to live in. Colour is also of limited use. Young goldfish tend to be bronze in colour, but will change to a more orange/gold colour within the first few months of life. Old fish will usually lose all of their gold colouration and be a creamy white colour.
If the goldfish is exposed to seasonal changes in living conditions, particularly temperature, it is possible to age them by looking at the scales under a microscope. The scales are made up of concentric rings which are formed closer together during cold weather when there is little fish growth, and further apart in warm conditions when the goldfish are growing more quickly. By counting the bands of closely spaced rings, the age of the fish can be determined. This is very similar to the process for aging tress, where the rings in a cross section of the trunk are counted. Even experts in scale reading find it difficult to age elderly fish accurately. In their later years fish tend to grow very little, so there is almost no gap between the scale rings – sometimes for many years.