How exact age is calculated
Age looks trivial and is not, because months have different lengths and February changes size every four years. The correct approach is to subtract calendar components in order — years, then months, then days — and borrow when a component goes negative, exactly like long subtraction.
Borrowing from the previous month rather than assuming 30 days is what makes the answer exact. Someone born on 31 January is 1 month old on 28 February in a common year, and the arithmetic has to reflect that.
Leap years and 29 February
A leap year is divisible by 4, except centuries, unless the century is divisible by 400. So 1900 was not a leap year and 2000 was. The rule keeps the calendar aligned with the solar year, which is about 365.2422 days long.
People born on 29 February — "leaplings", roughly 1 in 1,461 of us — have no birthday in three years out of four. Most jurisdictions treat 1 March as the legal anniversary for age-based rights, though a few use 28 February. This calculator counts the completed years, which matches the common-sense reading either way.
The ages that actually change something
Most of the time your age is trivia. Occasionally it is a threshold, and knowing exactly when you cross one is worth more than the number itself.
| Age | What changes |
|---|---|
| 16–18 | School leaving, driving, voting, legal majority — varies by country |
| 25 | Car hire without a young-driver surcharge in most markets |
| 50 | Catch-up contributions allowed in many retirement schemes |
| 59½ | Penalty-free withdrawals from US retirement accounts |
| 62–70 | The window for claiming Social Security; claiming later raises the payment |
| 65 | Medicare eligibility in the US; pension age in many countries |
| 73 | Required minimum distributions begin in the US |
Two places where an exact date genuinely matters:
- School cut-offs. Many systems use a fixed date, so a child born days apart can start a full year apart. Use the second date field to check age on a specific cut-off.
- Insurance “age nearest birthday”. Some insurers round to the nearest anniversary rather than the last one, which can move a life-insurance premium noticeably. Applying a few weeks earlier occasionally saves real money.
Age is not counted the same way everywhere
- Western reckoning — you are 0 at birth and gain a year on each anniversary. This is what the calculator uses.
- Traditional East Asian reckoning — you are 1 at birth and everyone gains a year at Lunar New Year, so a baby born in December could be "2" a few weeks later. South Korea officially moved to the international system in June 2023.
- Insurance "age nearest birthday" — some insurers round to the nearest anniversary rather than the last one, which can quietly change a premium.