Why your dog isn't the age you think

What I'm saying is not exactly correct and it's more complicated than this, but it's the best way to make it easy to understand.

Age is described as how your body deteriorates in time, aka, the ability of your body to produce cells as the older ones die

Basically when cells die the body produces copies of them, but they are not exact copies, so the more times these copies are produced, the more they are "different"compared to the original ones, for example their life span decreases.  So the more time passes the more the differ, until they eventually lose the ability to multiply.

Apparently this works differently in dogs.
I would assume that our cells divide and multiply at a slower rate, and this rate exponentially grows in time
While dog cells multiply at a faster rate when they are young, and it slowly slows down in time

As you can notice, our graphic is basically an exponential scale, while the dog's is the exact opposite, therefore a logarithmic scale

This also explains why it takes us ~20 years to properly develop biologically, while dogs are basically almost fully developed after 1 year already 
Since their cells multiply at a faster rate initially, they age faster at first