A goldfish should either be born with bubbles on the eyes, or not have them at all, they don't develop by age, they just grow along with the fish
But it is a problem having so many goldfish in such a small place, and since you switched the filter recently, you probably caused the tank to start a mini cycle, meaning that the ammonia is building up, making your fish weaker and susceptible to disease.
I had a bout with pop eye recently, a big water change was enough and it passed, turns out (as it said in one of my books) if it's just one eye on one fish, it could be a physical injury causing the blood vessels behind the eye to swell, and it would pass in a week
(Heh, breeding time in the tank, of course one of my little boys had to get bumped D: )
As everyone else said, some photos would probably help!