Why is one human year equal to seven dog years?

Question: Why is one human year equal to seven dog years?
- Keywords:
  biology,
  dog,
  human
Asked by Katie to Kieran on 7 Nov 2018.
- Kieran answered on 7 Nov 2018:
  
  Hi Katie, I am not sure why people say this. I would guess that an old dog is considered to be maybe 12 years. the average lifespan of a human is thought to be approximately 85 years, which is ~7 times that of a dog??
  
  Can you shed any light?

Question: Why is one human year equal to seven dog years?