Age is counted in whole years first, then whole months, then the remaining days, the same way it is counted on official documents. Total days count every calendar day, including leap days.
People born on 29 February celebrate on 1 March in years without a leap day here; some countries use 28 February legally.
Common questions
How is age calculated in days?
By counting the calendar days between the two dates, which includes the extra day in each leap year.
How old will I be on a future date?
Change the "Age on" date to any future or past day.