They're the Same Age

A grandpa, who was turning 72, and his granddaughter, who was turning 7, had their birthdays on the same day. The granddaughter told her grandpa that he was a lot older then her. Being older and wiser he said “No, we are the same age.” She said, “But grandpa, I am only seven. What number are you?” He replied, “Yes you are, and I am only seven too!”