回答
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At what age were you most happy?
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What age were you when you were the happiest?
この文章が以下のように翻訳されています。
何歳の時が一番楽しかったですか? ー At what age were you most happy?
何歳の時 ー At what age / what age were you when you were
が一番楽しかったですか? ー were you most happy? / when you were happiest? / were you happiest?
参考になれば嬉しいです。