Not exactly. The first year of a dog’s life is generally closer to 15 human years, and size and breed factor into how quickly dogs age. Smaller dogs tend to live longer than larger breeds, and they don’t become “seniors†until around 10 years, but they may mature faster in the first few years of life. A bigger dog might age more slowly in the beginning but be nearing middle age at 5 years. Medium-sized dogs fall somewhere in the middle.