The question arises early when we see how quickly puppies grow. We might also be surprised at how soon sexual maturity occurs.
Aging is different from one breed to another. As a general rule, a smaller dog develop faster, with some exceptions for some breeds.
Lifespan depends also on occurrence of diseases or accidents. Some breeds and gender are more exposed than others.
So, if we really want to get a good estimation of the age, and this matter is of real importance for providing good care to our dogs, we need to dig deeper.