The misconception among dog owners is that the dog gets 7 years older every year. In fact, dogs age much more quickly during the first 2 years of their life.
Below is a spreadsheet of age for each small, medium, and large dog size:
One of the best ways to prolong the life of your dog is to adjust the amount of food that is appropriate for each age. Older dogs are less active and require fewer calories.
However, many people still keep the same calories for each meal since their childhood, which is not good. Older dogs also reduce their ability to digest and absorb nutrients, so we need to choose high quality foods for them.
Excessive amounts of protein, phosphorus, sodium and can worsen kidney problems and heart problems in old age. On the other hand, the levels of vitamins, zinc, fatty acids, and fiber, should be increased with the life of the dog.