Koi reach about 50% of their final adult weight at 24 months, 95% at 10 years, and 99% at 14 years. Koi gain the most amount of weight at the age of 3 years when they can gain almost ¼ pound a month. The growth of the scales is proportional to the Koi’s growth and annual marks are formed on the scales at the same time every year along the outer edges. This means that a Koi’s scales tell its age, just like the annual rings in a cross-section of a tree.