nijuudai-sanjuudai-yonjuudai-gojuudai

2021年12月28日2026年4月21日

How do you say someone is “in their 20s” or “in their 50s” in Japanese? English uses a simple pattern — “twenties,” “thirties” — but Japanese has its own system using the word 代 (dai). Once you understand this pattern, you can express any decade of age naturally. This guide covers everything from the vocabulary to the cultural context around age in Japan.

Yuka

Hey Rei, what’s the best way to remember the difference between nijuudai-sanjuudai-yonjuudai-gojuudai and (‘it’, ‘that’)?

Rei

The best trick is to associate each word with a strong image or situation. By the end of this article you’ll have one for each — promise!

TOC

At a Glance: Age Decades in Japanese

Japanese	Reading	Meaning	Age range
10代	juudai	Teens / in one’s teens	13–19
20代	nijuudai	In one’s 20s	20–29
30代	sanjuudai	In one’s 30s	30–39
40代	yonjuudai	In one’s 40s	40–49
50代	gojuudai	In one’s 50s	50–59
60代	rokujuudai	In one’s 60s	60–69
70代	nanajuudai	In one’s 70s	70–79

The Pattern: Number + 代 (dai)

The word 代 (dai) means “generation” or “age decade.” When attached to a decade number, it creates the expression “in one’s ___s.” The pattern is completely regular: just say the number (20, 30, 40, 50…) followed by 代 (dai).

Formation note: 20 = 二十 (nijuu), so 20代 = nijuudai. 30 = 三十 (sanjuu), so 30代 = sanjuudai. The pattern holds for all decades.

Example 1 — stating your own age decade:

私は20代です。
Watashi wa nijuudai desu.
I am in my 20s.

Example 2 — talking about someone else:

彼は30代に見える。
Kare wa sanjuudai ni mieru.
He looks like he’s in his 30s.

Example 3 — describing a demographic:

このサービスは40代・50代に人気があります。
Kono saabisu wa yonjuudai / gojuudai ni ninki ga arimasu.
This service is popular among people in their 40s and 50s.

Yuka

That makes sense! So nijuudai-sanjuudai-yonjuudai-gojuudai is about… okay, I think I’m starting to get it.

Rei

You’re getting it! And the more you practice using nijuudai-sanjuudai-yonjuudai-gojuudai in sentences, the more automatic it becomes. Language learning is all about repetition.

Each Decade in Detail

20代 (nijuudai) — In One’s Twenties

The 20s are often called a time of freedom and self-discovery in Japan. Many Japanese in their 20s are recent university graduates starting their careers.

20代のころ、よく旅行をしていた。
Nijuudai no koro, yoku ryokou wo shite ita.
When I was in my 20s, I used to travel a lot.

30代 (sanjuudai) — In One’s Thirties

The 30s in Japan are often associated with settling down, marriage, career advancement, and family responsibilities.

30代になってから、健康に気をつけるようになった。
Sanjuudai ni natte kara, kenkou ni ki wo tsukeru you ni natta.
Since turning 30, I’ve started taking better care of my health.

40代 (yonjuudai) — In One’s Forties

The 40s in Japan are often called 働き盛り (hatarakizakari) — the prime working years, when many people are at the peak of their careers.

彼女は40代とは思えないほど若く見える。
Kanojo wa yonjuudai to wa omoenai hodo wakaku mieru.
She looks so young that you wouldn’t think she’s in her 40s.

50代 (gojuudai) — In One’s Fifties

The 50s in Japan often bring thoughts of retirement planning, children leaving home, and a shift in priorities.

50代で新しいことを始めるのは遅くない。
Gojuudai de atarashii koto wo hajimeru no wa osokunai.
It is not too late to start something new in your 50s.

Yuka

Alright. And now explain (‘it’, ‘that’)? I want to make sure I have both down.

Rei

Sure! (‘it’, ‘that’) is actually the easier one to remember once you have a clear mental image. Let’s look at the examples.

Useful Phrases with Age Decades

Japanese phrase	Romaji	Meaning
20代前半	nijuudai zenhan	Early 20s (20–24)
20代後半	nijuudai kouban	Late 20s (25–29)
30代に入る	sanjuudai ni hairu	To enter one’s 30s
40代になって	yonjuudai ni natte	Since turning 40 / after hitting 40
年齢は？	nenrei wa?	What is your age?
おいくつですか？	oikutsu desu ka?	How old are you? (polite)

Cultural Note: Asking Age in Japan

In Japan, asking someone’s exact age directly can sometimes feel invasive, especially in professional settings. Using age decades as a softer reference is common and polite. Saying 「30代の方」(sanjuudai no kata, “a person in their 30s”) is much more natural in many contexts than asking 「何歳ですか？」(nansai desu ka?, “How old are you?”).

Also note: Japan uses both Western ages (満年齢, man-nenrei) and traditional Japanese ages (数え年, kazoedoshi, which counts differently). In modern life, the Western system is standard.

Decision Flowchart: How to Express Age Ranges

Do you want to say someone is in a certain decade of age?
        |
        v
Take the decade number (10, 20, 30, 40, 50...)
        |
        v
Add 代 (dai) after it
        |
        v
Add に見える (looks like), です (is),
のころ (around the time of), などに人気 (popular among)
        |
        v
Examples:
20 → 20代 (nijuudai) → 20代です / 20代に見える
30 → 30代 (sanjuudai) → 30代のころ
40 → 40代 (yonjuudai) → 40代後半 (late 40s)
50 → 50代 (gojuudai) → 50代で始める