Answer:
When you're trying to figure out the year someone was born based on their age, unfortunately it isn't as simple as just minusing the persons age from the current year, though this is an easy way to get their approximate year of birth.
Just minusing someone's age from the current year doesn't take into account whether they have already had their birthday this year or not. To do that what you need to do is minus an additional one year if they haven't had their birthday this year.
