How it's calculated
birth year = reference year − age
reference year = year of the as-of date, age = age in whole years; result spans a 1-year range depending on whether the birthday has passed.
Common questions
Why does it give me two years?
Your age alone does not say whether your birthday has happened yet this year, so both possible birth years are shown.
How do I know which year is mine?
If your birthday has already come this year, use the earlier year. If it is still ahead, use the later one.