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。
この場合、次のような言い方ができますよ。
ーWhat age in your life was the most fun for you?
「人生で何歳の時が一番楽しかったですか?」
ーWhen did you have the most fun in your life?
「人生でいつが一番楽しかったですか?」
ご参考まで!