A surprising number of pet parents don’t know when their pets become seniors. To convert your pet’s chronological age to the equivalent in human years, start with 15 human years when your pet is a year old. This is based on the age at which they achieve sexual maturity. When your pet turns two, add nine years, so that at age two, your pet is the human equivalent of 24 years old. After that, simply add four “human“ years for every pet year.