Pets age much faster than we do. The life span of a dog depends on its size or breed. In general, the larger the breed or size of the dog, the shorter the life span. For example, in a study of lifespans, only 13% of giant breed dogs lived to be over 10 years old. Conversely, 38% of small breed dogs live to be over 10 years of age.
Dogs are considered senior in the last 25% of their lives. Below is a list of the most common breeds with their life expectancies and age at which they are considered "senior".
Papillon: Lifespan: 12- 15 years; Senior years: 9-11
other breeds: http://www.petplace.com/article-printer-friendly.aspx?id=5660
(All copied from the above link)