Find out how old a wild mouse is by observing it closely in its native habitat. The age of a newborn mouse will be easier to identify based on its lack of fur and other infantile features including unopened eyes. But even older mice can be identified with a few clues to what parts of its body are developing and the thickness of its fur. Bring along a quarter to help measure the mouse and determine the day or week of its age.
Look at the features of the mouse. Mice with their eyes closed are probably just a few days old. Newborn mice will have visible pink skin and no fur on their bodies. Their paws will still be extremely small and almost appear webbed. A mouse will start to grow small nubs for ears around the second or third day.
Measure the approximate length of the mouse against a quarter. Mice that cover the approximate length of a quarter and a half are generally a week old or more. Look for signs of fur growing in and definition between its claws on its paws for other signs that the mouse is at least a one week old. A mouse completely covered in fur will be older than a week. Do not count the tail as part of your measurements.
Examine the mouse's ears. Once they are fully formed and extend from the head, the mouse is at least five days old. You should also notice the mouse's skin seems thicker to the touch and fuzz is appearing around its neck. When colored fuzz appears, you will know the mouse has reached at least one week old.
Look to see if the mouse's eyes are opened yet. A mouse is at least 13 days old when its eyes open. You will also notice a thicker coat of fur, lower and upper incisor teeth showing, and nipples on females by the time the mouse reaches two weeks old.
Get out your quarter again and measure it against the mouse's head. The quarter and the mouse's head should be approximately the same length by the time the mouse is 28 days old or more. By now the mouse has a full coat of fur, all teeth are showing, a female's nipples are fully developed, and eyes are open and clear.