It can be difficult to pinpoint the age of a baby squirrel if you didn't see it born. However, there are a few ways to judge approximate age using photos. The amount of hair a squirrel has, how its ears look and even its level of playfulness in a photo all provide clues.
Look at the amount of hair on the squirrel. The newborn is pink and practically hairless. Its hair starts growing in right after it's born but, according to squirrels.org, the hair doesn't start to show until a squirrel is about 2-1/2 weeks old. The hair will start to look more full and complete when it's around 6 to 7 weeks old.
Look at its eyes. Are they open yet? If they aren't, it's younger than a month old. The eyes start to open at around a month to 5 weeks old.
Look closely at the perkiness of its ears. If the ears are still a little bunched up, the squirrel is most likely less than a month old. The ears start to open and stand up at around 4 weeks of age.
Look at what the squirrel is doing in the picture. If it's eating something with its hands, it could be at least 7 to 8 weeks old; it starts foraging for food around that time. If it's playing with another squirrel, chances are it's at least a couple of months old; this is when it's developing its social skills.
Check, if possible, for any teeth. The lower ones start growing in its third week of life. The upper incisors start to be visible at 4 to 5 weeks old.