It is very possible that your fish are getting ready to breed. Not the right time of the year but I presume your tank is inside so the fish are subject to unnatural light and temperature cycles. Just leave them as this will soon pass. If you're interested in breeding, have a go. It is fun and very interesting.
When a female goldfish feels her eggs are ready to spawn she releases a hormone in the water that signals the males to get into breeding condition. In the final few days the males go berserk and chase her into the plants to test if she is ready to breed. This is the 'sniffing' you are observing. When ready she will release a dozen or so eggs when pushed into the plants and the successful male will quickly release its milt over the eggs thus fertilising them. The bigger the female the more eggs. A 6" fish may release over 1000 eggs.
In nature the fish will just go their merry way after breeding and repeat the procedure 2 or 3 times per year in other parts of the lake/stream. In captivity they can not go their merry way and are stuck in the tank. Once the hormones are gone they revert to their second most important natural activity - eating. The eggs will make tasty morsels and depending on the plants and tank size, most would be gone in the morning. If the fish are removed straight after breeding the eggs will hatch in 2 to 7 days and 3 days later the fry will require brine shrimp hatchlings to do well. As they grow, daphnia and tubifex should be provided as well as gradually increasing the dried food.
The size of the fish is not a clear indicator of the age. There are many different breeds and many 'pet shop fish' are of a mixed heritage. It also depends on how much room the fish had when they were growing actively, crammed conditions mean stunted fish.