The older and simpler method of converting dogs and cats ages to human years uses a multiplier of 7, so for instance, a 5 year old dog is 5×7 or 35 in human years.  But as pets get into the 15 plus years this formula is not so accurate.

A more realistic equation is 10 human years for each pet year up to 3 years of age, then 4 human years for each year thereafter.  That would make a 15 year old pet [15-3] x4 +30 or 78 years old.