The 接尾後（せつびご） (suffix)
～代（だい） is used for several different things. One of the most common uses is for highlighting someone’s decade of age. In the same way that English says teens, twenties, and thirties. All that is required is adding
代（だい） after the age (in tens), in Japanese.
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10代（だい）の頃（ころ）によく怒（おこ）られたよ。
I was scolded often when I was in my teens.
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私（わたし）は20代（だい）の頃（ころ）に沢山（たくさん）旅行（りょこう）をしました。
I traveled a lot in my twenties.
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六十（ろくじゅう）代（だい）の方（かた）はこちらにお並（おなら）びください。
People in their sixties, please line up over here.
In addition to someone’s decade of age,
～代（だい） can be used to express a chronological decade, such as the 1960’s, 70’s, or 80’s. In these cases, 年（ねん）
代（だい） will be used after the decade.
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70年（ねん）代（だい）の音楽（おんがく）が好（す）きです。
I like music from the 70s.
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80年（ねん）代（だい）のファッションはかっこいい。
Fashion from the 80s is cool.
代（だい） is also regularly seen when marking eras of time, or generations of people within a specific family/political position.
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新生（しんせい）代（だい）は哺乳類時（ほにゅうるいじ）代（だい）とも言（い）われています。
The Cenozoic era is also known as the era of mammals.
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第（だい）5代（だい）将軍（しょうぐん）徳川綱吉（とくがわつなよし）はとても有名（ゆうめい）な将軍（しょうぐん）です。
The fifth shogun, Tokugawa Tsunayoshi, is a very famous shogun.