Your snake is an adult or at least a sub-adult. There's no way to tell how old. It could be 2 years or it could be 10 years.
Full grown corn snakes can range in length from 3 feet to 6 feet, although the majority are around 4 to 5 feet.
I prefer to speak in terms of weight when talking about feeder mice. Because what some people call a "Medium Mouse" other people may call it a Hopper. Some companies package mice as Small, Medium, Large and Jumbo, and it can vary from distributor to distributor and can be confusing.
You stated you feed that snake a Medium mouse, but what does that really mean?
My snake is much smaller than your's (about 210 grams) and he's eating mice that weigh 20 to 25 grams. They are packaged as "Large Mice" where I buy them. Your snake should be eating mice bigger than that. You stated your snake weighed 450 grams. If that was my snake, I'd be feeding it at least a 30 gram mouse (regardless of what you want to call it).
I have always fed snakes based on weight. Some people think that's confusing. Mathmatically challenged folks I guess, but it's quite simple if you have a scale. You weigh the snake, you weigh the mouse. You feed a mouse that weighs about 10% of the snake's weight. If you can't find a mouse that big you opt for a Chick or a Rat. When the snake is young and still growing, you feed weekly, and once it's full grown you feed every 2 or 3 weeks so the snake doesn't get fat. I have been feeding snakes like this for over 40 years and never had a health issue related to feeding, never had a regurge, never had a snake die other than from old age.
I would say your snake is certainly nearing full grown status.