He realized his ambition to climb Mount Everest before turning forty.

realized 是动词 realize 的过去式形式。realize 由词根 real（真实的）和动词后缀 -ize（使...化）组成，原意是"使成为现实"，引申为"实现、认识到"的意思。在这里表示"实现了（抱负）"。