How the age is counted
Whole years first: the birthdays that have passed. Then whole months since the last birthday, then the days left over. Someone born on 15 March is one month older on 15 April, however many days April's month had. This is how ages are counted on forms and in law, so the answer matches what you would work out on a calendar.
A date other than today
Change Age on to find how old someone was, or will be, on a particular day: the date of an exam, a wedding, or the day a rule about age applies. It can be a date in the past or the future.
Born on 29 February
In a year without 29 February, the birthday is shown on 28 February. Some places count the day after, 1 March, as the legal birthday in those years, so check if it matters for a document.
Questions
Why does the total in months not match years × 12?
It does: the total adds the months since the last birthday to the years × 12. The total in days is counted exactly, leap days included.
Can I work out the time between any two dates?
Yes. Put the earlier date in Date of birth and the later one in Age on; the result is the time between them.
Is my date of birth stored?
No. It is worked out in this page and goes nowhere.