Summarize this article:

If you have a puppy, it's fairly easy to determine its age by examining its teeth. Teeth start developing during a puppy's first year. The stage of teeth development is tied to a puppy's approximate age.  At one month old, teeth will have just started pushing through the gums. At two months, a puppy will have a full set of deciduous (baby) teeth near the front of the mouth. At four months, adult canines and incisors will start to come in. By seven months, a puppy will have a full set of adult teeth and will have developed molars in the back of the mouth. When a puppy reaches a year old, you should notice ridges or bumps on both the lower and upper top front teeth. Keep in mind that teeth tend to erupt later in toy dog breeds. As a dog ages, their teeth change. Changes in teeth can help you assess a dog's approximate age.  Ridges on the incisors change as a dog ages. By the time the dog is three years old, the ridges will have worn off the bottom incisors. By five years old, the ridges will have worn off the upper incisors as well. A small amount of ridges on the upper front four teeth indicate your dog is between three and four. By seven, a dog's teeth are completely smooth. Older dogs tend to be less energetic than younger dogs. Senior dogs often have small benign lumps on the body. They will also be less interested in playing than younger dogs. They are generally less muscular than younger dogs. Cloudy eyes are a sign of natural hardening of the lens as the dog ages, and this is called lenticular sclerosis. If your dog shows other signs of aging, such as smooth teeth and low energy, lenticular sclerosis can help you confirm your dog is older than seven. Lenticular sclerosis and cataracts are not the same thing. Cataracts are white and opaque and they are also common in senior dogs. However, some dogs develop cataracts young due to genetic reasons. Cataracts will affect your dog's vision, whereas lenticular sclerosis will not. Fur color often changes as a dog ages. However, fur color is not the best means of determining age. However, when taken into account with other factors like teeth, fur may help determine a dog's age. Most dogs have some graying fur past the age of five. Graying hair is typically found around the muzzle. Some dogs gray as early as two, depending on their environment and genetic makeup. Fur color can help confirm a dog is older when taken into consideration alongside other factors, like teeth and activity level. It should not be used as the sole means to determine a dog's age. Larger breeds tend to have shorter lifespans. They may show signs of aging younger than smaller dogs. Great Danes, for example, typically live around seven years while Chihuahuas often live well into their teens. A bigger dog may show signs of aging early. In general, you can assume a big dog showing significant signs of aging is around seven or eight while a small dog can be assumed to be ages 10 or up.

Summary:
Look to teeth to determine a puppy's age. Note changes in teeth for an older dog. Look for signs of aging. Examine your dog's eyes. Use fur color to determine age. Take your dog's size and breed into account.