Why "total days ÷ 365.25" isn't how real age is expressed
People express age as "X years, Y months, Z days," not as a single decimal number of years — a calendar-accurate age calculation reflects this by counting the largest whole number of complete years between the two dates, then the largest whole number of complete months from there, then whatever days remain. This matches how a birthday actually works (you turn a year older on a specific calendar date, not on a rolling 365.25-day anniversary).
A worked example
Someone born 15 June 1990, as of 13 September 2026: 36 years, 2 months, and 29 days old (13,239 total days, 434 total months). The year count comes from the most recent birthday having occurred (15 June 2026), the month count from how many complete months have passed since that birthday (up to 13 August 2026), and the day count from the remaining days to 13 September.
The genuinely tricky case — a leap-day birthday in a non-leap year
Someone born on 29 February 2000 (a leap day), evaluated as of 28 February 2026 (a non-leap year, so 29 February doesn't exist that year): the calculation gives exactly 26 years, 0 months, 0 days — treating 28 February as the effective "anniversary" date when the exact leap-day date doesn't exist in the current year. Evaluated one day later, on 1 March 2026, the same person is 26 years, 0 months, 1 day old. This means a leap-day baby's calculated "birthday" lands on 28 February in ordinary years, one day earlier than in the rare years when 29 February actually exists.
Why this month-end handling matters more broadly
The same underlying logic — clamping to the last valid day of a month when an exact calendar-date match doesn't exist — also governs less extreme cases, like counting months from 31 January: since not every month has 31 days, "one month after 31 January" clamps to the last day of February rather than overshooting into March. This is a general and necessary rule for calendar-accurate date arithmetic, not something specific to leap-day birthdays alone.
Why this precision matters beyond curiosity
Many real eligibility rules are stated in exact years and sometimes months (minimum age for a scheme, a specific age-based tax exemption, eligibility bands with month-level cutoffs) — a calculation that's off by even a day or two around a birthday boundary can genuinely change an eligibility outcome. Calendar-accurate age calculation, done consistently including these edge cases, is what makes an age result reliable for such boundary-sensitive uses, not just for casual curiosity.