A dog does not get old as a human being. A very widespread but false method consist to multiply 7 times the age of the dog to find the equivalent in "human years".
The age calculated this way is totally wrong. Actually, it depends on the age and the size of your dog.
Thus, the youth of dogs is very short compared to its maturity and old age. During the 1st year, a dog can grow up to 20 times faster than a man!