Puppyhood generally ends sometime between 6 and 18 months old, depending on the dog's breed. Adulthood can start anytime between 12 months to 3 years. Small dogs usually mature earlier and live longer than large dogs, while large dogs have longer periods of puppyhood and adolescence.
Regardless of breed, a dog's life cycle is broken down into four stages: puppyhood, adolescence, adulthood and senior years. A dog enters the last phase of its life between 6 and 10 years old, though it may live considerably longer than that.
The period between puppyhood and adulthood is a rebellious adolescent phase during which a puppy often tests its boundaries and limits, seeking to feel confident and secure in its role. The dog may require constant supervision during this period and may need its training to be reinforced consistently. During this stage, the dog needs to be socialized with other dogs, people and children. If not neutered, the dog may begin displaying interest in dogs of the opposite sex. A female dog first goes into heat around 5 or 6 months of age, and a male dog's testosterone levels rise during adolescence. After about 10 months, a male dog's testosterone level settles.
A dog is generally considered an adult once it is 1 or 2 years old, but its social maturity may take more time to develop, depending on how often it is exposed to other dogs and humans.