Because human lifespan was regarded as 50 years back in the day, today, you are deemed as longevity if you are sixty years of age and older.
こんにちは！
ご質問ありがとうございます。
『昔は人生50年と考えられてて還暦以上は長寿とみなされてる』は、いくつか言い方が考えられますが、例えば、
Because human lifespan was regarded as 50 years back in the day, today, you are deemed as longevity if you are sixty years of age and older. と言えます。
メモ
human lifespan 人間の寿命
back in the day ずっと前、昔々
deemed as と見なされる
_ years of age 年齢＿歳
参考になれば幸いです。
日本 
