Of course.
I can age a fish very easily, without even having to compare them. You can use the fin rays, scales and bones to determine the fishes age. They all have annual growth rings, similar to the rings of a tree. But the most accurate way to age a fish (especially older ones) is by examining the otoliths, or 'earstones'. They are located in the inner ear, directly below the brain. They serve much the same function as the inner ear bones (hammer, anvil and stirrup) in humans. In a bluegill, they are a little smaller than a pea, and are free-floating in the soft tissue of the inner ear.
The procedure is as follows: you will need a dissecting microsope (mine is a Nikon, they are the best IMO), a Bunsen Burner, or candle, forceps, a small piece of clay, alcohol, and some vegetable oil.
1. Remove the otoliths (there are two) and clean them well with alcohol.
2. Next, split one of them in two along the center-line by placing the otolith on the pad of your index finger, convex side up. There is a groove along the top called the sulcus. Place your thumbnail in the sulcus and apply firm downward pressure until the otolith snaps in two (be careful, it can take a bit of pressure, and the pieces can go flying. They are hard to find, so do this over a good work surface. If it breaks too far from the centerline (it happens rarely), it cannot be aged. If this happens, just repeat the steps with the other otolith.
3. Some very talented scientists can read the annuli (growth rings) with just a light coating of oil, but I never could. If you are like me, burn the otolith piece slightly with the Bunsen Burner, or candle, then coat it with oil. This makes the annuli much easier to see. To burn it, light the burner and let it burn for a bit so you have a nice, clean, soot-free flame. Next. hold the otolith half with forceps and hold it vertically, facing you, about 1/2" above the flame, until it starts to turn brown (usually just a few seconds). Set it down on a clean surface to cool for a few minutes, before trying to handle it again.
4. When the otolith half is cool, place it cracked-face-up in a small piece of clay and light press it down. Place a small drop of vegetable oil on the face of the otolith and gently spread it evenly. Now, place the otolith under the microscope. Use a 10X magnification. The annuli will be visible as evenly-spaced brown or black lines. Count the evenly spaced lines nearest the edge (many are not annual and will appear wider and less regular, and will be nearer the center). This is how old the fish is.
It takes a little practice, but it is absolutely accurate, one you get good at it.