Age is systematically rounded down: in many countries, a human is age zero for the first year of their life and increments to being one year old only after they have finished that whole period of their life. You are always older than your age. When you are thirty-nine, you are not in your thirty-ninth year of life but your fortieth. If you count the day of your birth as a birthday (which is hard to argue against), then when you turn thirty-nine, it is actually your fortieth birthday. True as that may be, in my experience, people don’t like it written in their birthday card.
