I bought all my goldfish when they were under 2 inches long. From my experience, that means they would have been between one and three months old, since most feeders are raised in ponds or large tanks with ample growing room and ample food. That's how I know how old they are. If you buy goldfish when they are older, from an LFS rather than a breeder, then it can be much more difficult to determine age. Size will vary widely depending on environment and genetics.
Breeders usually keep track of the age of their fish, so buying from a breeder has more benefits than just good broodstock.
Breeders usually keep track of the age of their fish, so buying from a breeder has more benefits than just good broodstock.