Age in Japan: full age, kazoe-doshi, and the "day-before-birthday" rule

In Japan, “full age” (満年齢) is standard but “kazoe-doshi” (数え年, “counted age”) survives in tradition. And by law, age increments at midnight before your birthday — which has surprising consequences. This article explains.

Full age vs kazoe-doshi

Full age

Years actually elapsed since birth:

• At birth — 0.
• First birthday — 1.
• Thirtieth birthday — 30.

Modern Japan’s standard (formalized in the 1950 “Law on the Counting of Age”).

Kazoe-doshi (counted age)

You’re 1 at birth, and everyone ages on January 1:

• Born — 1.
• Next January 1 — 2.
• Thirtieth birthday — full age 29 or 30, kazoe 30 or 31.

The notion that “everyone ages together on New Year’s Day”.

Legal age increment

Civil Code Article 143 / “Law Concerning Age Calculation”:

Age increments at 24:00 on the day BEFORE the birthday

Example, born January 1, 1990:

• Legally turns 1 at 24:00 on December 31, 1990.
• “End of day before” = “start of birthday” — they’re the same instant.

Consequences:

• April 1 birthdays are “early-year” (前年度) school cohort, ahead of April 2.
• Born April 1 → reaches age N at 24:00 on March 31 / 0:00 on April 1.

An April 1 birth is a school year ahead of an April 2 birth.

Early-birthday cohorting

Japan’s School Education Act admits to elementary school the children “who turned 6 by April 1”:

• Born April 1 → turns 6 at 24:00 March 31 → enters school that April.
• Born April 2 → turns 6 at 24:00 April 1 → enters school the next April.

So:

• Born January – April 1 — “early year” (一緒に進級).
• Born April 2 – December — “late year”.

In sports and entrance-exam competition, “early-year kids” are often disadvantaged because they share a cohort but are up to 12 months younger.

February 29 birthdays

Leap-day birthdays under Japanese law:

• Article 143 — age increments at “24:00 on the day before”.
• Non-leap years — increments at 24:00 on February 28.
• Leap years — also 24:00 on February 28 (the day before Feb 29).

So leap-day-borns reliably age at 24:00 on February 28 every year.

By country:

• Japan — 24:00 on Feb 28 (Civil Code).
• US/UK — by jurisdiction (Feb 28 or Mar 1).
• Taiwan — March 1.

Where kazoe-doshi still appears

Modern Japan uses kazoe in some traditions:

• Shichi-go-san (3, 5, 7-year-old festival) — depending on region, full age or kazoe.
• Yakudoshi (unlucky years) — male 25, 42, 61; female 19, 33, 37 (typically kazoe).
• Long-life celebrations (kanreki at 60, beiju at 88, etc.) — mixed.

The “year-of-the-zodiac” culture also tilts toward kazoe.

South Korea pre-2023

Korea historically used “Korean age” (한국 나이) — kazoe-style:

• Born — 1.
• Everyone ages on January 1.

A baby born December 31 became 2 the next day (actual age 2 days).

June 28, 2023 reform — switched to full age (만 나이) by default. There was a transition period of mixed usage in conversation and news.

China and Vietnam

Traditional East Asian “xū suì” (虚歳, China) and “tuổi mụ” (Vietnam):

• 1 at birth.
• Increments at lunar new year.

Daily use is full age now, but rituals still use the traditional count.

Computing age in code

JavaScript:

function calculateAge(birthDate, now = new Date()) {
	let age = now.getFullYear() - birthDate.getFullYear();
	const monthDiff = now.getMonth() - birthDate.getMonth();
	if (monthDiff < 0 || (monthDiff === 0 && now.getDate() < birthDate.getDate())) {
		age--;
	}
	return age;
}

calculateAge(new Date('1990-01-15')); // e.g., 35

Subtract 1 if “this year’s birthday hasn’t arrived yet.”

Legal milestones (Japan)

Age	Legal change
13	Age of consent (Penal Code 177)
14	Criminal responsibility (Juvenile Act)
15	End of compulsory education
16	Marriage age (women, harmonized to 18 in 2022 reform)
18	Adulthood, marriage (both), voting, driver’s license
20	Drinking, smoking, gambling on horse / bike races
25	Eligible for House of Representatives
30	Eligible for House of Councillors, prefectural governor
65	Senior status, pension start

The 2022 Civil Code reform lowered adulthood from 20 to 18.

Strict-interpretation traps

“Turns 18 on April 1, 2024” parsed legally:

• Common reading — at 0:00 on April 1, 2024.
• Legal — someone born April 2, 2006 turns 18 at 0:00 April 1, 2024 (= 24:00 March 31).

So “born on April 1” turns 18 the day before, legally. Practical use usually treats the birthday itself as the threshold.

Western vs Japanese-era years

Japan also uses imperial era years:

• Showa 65 (1990) → 35 in 2025.
• Heisei 5 (1993) → 32 in 2025.

Computing in CE / Western years is less error-prone. Era years are best left as a display layer.

Summary

• Legal age in Japan is full age (Civil Code 143).
• Increments at 24:00 the day before the birthday.
• Early birthdays (Jan – Apr 1) are in the prior school cohort.
• Kazoe-doshi — born at 1, increments on January 1.
• Korea also moved to full age (2023).

To compute full age from a date of birth, the age calculator on this site handles the standard rules.