Age Calculator

For age in completed years, the calculation is straightforward: today's year minus birth year, minus 1 if today is before the birthday this year, plus 0 if today is on or after. Example: born March 15, 1990. On March 14, 2025: still 34. On March 15, 2025: turned 35.

For exact age in years/months/days, walk through the math: years = current_year − birth_year (with the birthday-not-yet adjustment); months = current_month − birth_month + 12 if the result is negative (with a year decrement); days = current_day − birth_day + days_in_previous_month if negative (with a month decrement). The result is the human-natural "35 years, 4 months, 12 days" form.

Someone born on February 29 has only one true "birthday" every 4 years. Most jurisdictions treat their birthday as February 28 in non-leap years for legal purposes (turning 21, voting age, etc.), but a few countries (notably some U.K.-influenced jurisdictions historically) used March 1. For age calculation, the practical answer is to treat February 29 as February 28 in non-leap years — the same convention used by most software.

The leap-year rule itself: divisible by 4 is a leap year, except divisible by 100 isn't, except divisible by 400 is. So 2000 was a leap year, 1900 wasn't, and 2100 won't be. Most everyday age calculations don't span century boundaries; for those that do, the rule matters.

Age in years rarely depends on time zone — a birthday is a date, not a moment. Age in seconds or hours does: someone born at 23:30 in Sydney was born at 13:30 UTC; their "second of birth" anchor depends on which clock you use. For most purposes, treating the birth date as a calendar date in the person's birth-time-zone is correct.

Age in legal contexts — turning 18, 21, or 65 — sometimes uses the prior-day convention (the common-law "turning age on the day before the anniversary" rule). U.S. legal practice has largely abandoned this in favor of the anniversary-day rule, but specific statutes occasionally invoke the older convention. When a date matters legally, check the specific jurisdiction.

Federal U.S. ages: 16 (driver's permit/license, varies by state), 18 (vote, military service, contract eligibility, age of majority in most states), 21 (alcohol purchase nationally, casino gambling in most states), 25 (rental car at most companies without surcharge), 26 (loss of dependent status on parental health insurance), 50 (catch-up retirement contributions begin), 59½ (no penalty on retirement-account withdrawals), 62 (earliest Social Security retirement claim, with reduction), 65 (Medicare eligibility), 67 (Social Security full retirement age for those born 1960+), 70 (maximum delayed retirement credits stop accruing on Social Security), 73 (RMDs begin under SECURE 2.0).

Each milestone has specific rules — Medicare requires enrollment, RMDs require action by year-end, Social Security claim age affects monthly benefits permanently. An age calculator that flags upcoming milestones can be more useful than just reporting current age.