It was a defining moment.

Meaning

An experience that significantly shapes one's character or future.

Examples

• John: Graduating from university changed my life.
• Alice: It must have been. <b>It was a defining moment</b>, I bet.
• John: Absolutely. It shaped my career path and my entire outlook.
• Alice: I understand completely. For me too; it was a graduation that was <b>a defining moment</b>.