The old human-to-dog-years formula (one year of a dog's life is equal to seven years of human life) never really worked quite right. It's more accurate to think of the first year of dogs' lives as just getting them over the first big jump of adolescence. The second year brings them close to full adult maturity physically, although as in humans, mental maturity may still be on the horizon.

From there, it really depends on the kind of dog in question. Size and breed matter when it comes to aging. Roughly put, you can consider Great Danes to be senior citizens at 7, but Chihuahuas may not reach that milestone until they're at least 11.

Use the dog years to human years chart below to figure out how old your dog is.