遅生まれ

おそうまれ
born after April 1 (school entrance date)

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おそうまれ【遅生まれ】

being [a child] born on or after April 2 (and therefore starting school one year later than children born earlier in the year). [⇒はやうまれ]

●彼は遅生まれですから, 年齢は一緒でも学年は私より一つ下です. He was born after April 1, so even though he's the same age he's one year behind me at school.