While we all know that pets age faster than their owners, it is not always clear how much. Conventional wisdom has typically estimated pet aging at about 7 years per year of human life. This rule-of-thumb becomes obviously inadequate for cats and large dogs.
Many factors come into play in determining lifespan. Species, breed, size, heredity, diet, spay/neuter status, veterinary care, and lifestyle (such as, indoor or outdoor for cats) all factor into length of life. In dogs in particular, size matters. Big dogs don't live as long. Small dogs may not be "senior" until 10-12 years old, while giant breeds may be "senior" at 5 years old. Routine physical examinations, proper nutrition, preventative care, and appropriate senior care are the best ways to tip the lifespan balance in our favor.
The chart below is a rough guide to consider your pets real age.