Hi and welcome to the forum
The fish could be 12 months old or it could be 10 years old. It depends entirely on how big the environment was that the fish grew up in, how much food it got, water quality, and whether it got any diseases. All of these can affect the growth rate of fish.
Plecos that are 12 inches long are usually over 2 years old.
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If you post a picture of the fish we might be able to id it for you. Set your camera's resolution to its lowest setting so the images fit on the site. Use a flash and have lots of light in the room and on the tank. Angle the camera so the camera flash reflects away from the camera and not straight back at the lens.
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Monitor the ammonia, nitrite and nitrate levels over the next month and keep feeding down. If the fish looks off colour or you get any ammonia or nitrite readings above 0, do a 75% water change and gravel clean the substrate every day until the levels are back on 0.