(Kingussie. Inverness-shire, Scotland)
What age are goldfish when sold at about 3inchs long?
When giving a Goldfish's size, you need to quantify it a little further by adding whether the measurement is body length or total length.
There are so many fin variations, I only give body length measurements throughout the website.
Many enthusiasts discuss their 6 inch Comets, but in reality they are 3 inch with 3 inch tails. Fancy varieties grow outwards as much as in length, so a 3 inch Veiltail is likely to be 12 months old, but a Comet can achieve that size in a few months.
So I will make a few assumptions to answer your question.
I will assume 3 inches is body length, and the fish are the normal Common/Comet body type.
I don't know if the fish you are referring to are imported or bred and raised in Scotland.
The growing season isn't very long, so I suspect the minimum age
would be 12 months old (or last season's fry). If they have been stunted, they could be older.
Look for how developed the breeding tubercles are on the males. Young fish tend to have small well defined tubercles where older fish have larger clumps along the leading edge of the pectoral fins.
Young fish in good health should be active and looking for food, with no splits or damage to the fin edges.