A shih tzu reaches its full growth potential between the ages of 6 and 8 months but is not considered an adult until it is 1 year old. Adults stand between 9 and 11 inches from shoulders to toes and weigh between 9 and 16 pounds.
Despite its diminutive height, a healthy adult shih tzu appears sturdy and strong. While smaller sizes are available, the American Shih Tzu Club and the American Kennel Club both refuse to recognize imperial, teacup or stained glass shih tzus, citing their smaller sizes as a reflection of improper breeding methods and health problems. A larger shih tzu may have a different breed in its ancestry and may not qualify as a purebred.
Shih tzu puppies are popular choices, but some breeders recommend choosing an adult because this type of dog matures slowly. In light of this, the ASTC advises allowing puppies to remain with their mothers until they are 12 weeks old. An adult shih tzu needs a daily walk and regular play sessions and is at risk for developing behavioral disorders if its needs are not fully met. Shih tzus can live for up to 15 years and are considered senior dogs by age 8.