He died before his time. / He passed away before his time.
He is gone too soon. / He was gone too soon.
He died before his time and I'm still mourning him.
上の表現は「died young」や「died at a young age」と同じように「そんなに早すぎて死ぬはずはなかった」という意味なんですが、いつも年齢との関係があるわけではありません。例えば、成長して大人として生活したり、もう若者じゃなくてもまだ健康的な人がいきなり亡くなってしまう場合にも「died before his time」とか「gone too soon」とか言えます。