Why your dog isn't the age you think

Is this really a surprise to people? I think it's rather obvious if you have ever lived with a dog. They enter puberty in one year, exit puberty in two years and enter middle age around 8-10 years old and then are old for the rest of their lives. My dog who died at 18 was elderly for over half of her life. My current dog is about to turn 9 and entered his middle aged man phase this year, he probably has 6 or so years left in his life, so in a couple of years I'm expecting him to turn into a grumpy old man. A working dog usually retires around the age of 10, which I think has been pretty much the threshold into the elder hood for all the dogs I've known.  

I never, even as a kid, thought there was anything behind the "one human year is 7 dog years" besides to remind of the fact how much shorter a dog's life is when compared to humans. Although I do believe my parents might have explained that to me when I was in elementary school. 

And what they are measuring is deterioration, just like in humans.