If only you could ask a koi fish how old it is.
Generally speaking, larger koi are older koi, but length varies depending on the fish's exact breed, diet, health and genetics. (Contrary to myth, however, koi do not stop growing to accommodate the size of their pond or tank - so make sure they have ample space.) You won't be able to tell an exact age unless you look at nitty-gritty details like the size of the koi's ear bone or number of tiny rings on some of its scales.
When you buy koi from Splash, we can tell you that our smallest koi are probably around a year old and koi labeled as medium are about 2-3 years old. Splash also stocks larger koi and some Premium koi that are typically 3-5 years old.
In the right ecosystem, koi typically have a lifespan of 30 - 40 years, so those big, gorgeous koi in your pond could still bring you decades of enjoyment.
The oldest koi in modern records was a chagoi named Hanako. Hanako, meaning “flower girl” in Japanese, was born in 1751. (That's before the French Revolution and five years before Mozart was even born!) In 1966, Dr. Komei Koshihara revealed that pond where Hanako spent most of her 200 year old life in a quiet pond with a tiny stream of fresh water flowing in a small valley near Mt. Ontake in Japan. She died in 1977 at the age of 226 years old.