A dog's life cycle begins at puppyhood and spans an average of 12 years, although some dogs live considerably longer. The female dog generally become sexually mature by age one and can begin reproducing after that time if not spayed.
Most dogs are pregnant for 63 days and can have as many as 15 puppies in one litter. Smaller breeds generally have three puppies or less, while larger dogs may give birth to more than a dozen pups. Puppies are always born with their eyes closed; their eyes will not open until they are around two weeks old. Puppies grow quickly after birth, and generally only suckle the mother for about six weeks, during which time they begin to develop permanent teeth. Most dogs have reached their full growth potential by age two.
Puppies are generally considered to be adults by age two. Female dogs go into estrus (heat) twice each year, usually for about 10 days. The female emits an odor that is attractive to the male and that lets him know she's ready to conceive.
The world record for the oldest dogs stands at 29 years and 5 months. It belongs to an Australian cattle dog named Bluey. Bluey was born in 1910 and died in 1939.