How it is worked out
age = years, months and days elapsed from birth to today
Why it is done this way
Age is a calendar calculation, not an arithmetic one. Months have different lengths and leap years add a day, so the only correct method is to step forward month by month and then count the remaining days. Dividing total days by 365.25 gets close but drifts, and it cannot tell you the months and days.
Step by step
- Count the whole years from your birth date to today
- From that anniversary, count the whole months that fit
- Count the days left after the last whole month
- For the total in days, count the calendar days directly
What is worth knowing
The month-by-month method is why your age in months is not your age in years times twelve plus a bit: the remainder depends on which months you passed through. There is one genuinely ambiguous case, and the law does not agree with itself about it: someone born on 29 February has no birthday in common years. Most legal systems treat 1 March as the anniversary, some use 28 February, and the difference matters for anything that turns on an exact date of majority.