How is Opportunity Age calculated in reports?

Age is calculated as follows

• Until the Type of the Opportunity Stage becomes Closed/Won or Closed/Lost, the number of days from the Created Date to the present is counted.
• For Opportunity records where the Type of the Stage is Closed/Won or Closed/Lost, the number of days from the Created Date to the Close Date is counted.

*Note: You can check which Type each Stage value is categorized under in the Setup menu. As a System Administrator, please check by clicking Stage from the list of fields displayed in the following steps:

Salesforce Classic: 
Go to Setup > Customize > Opportunities > Fields

Lightning Experience:
Go to Setup > Object Manager > Opportunity > Fields & Relationships

If you wish to know the number of days from the Created Date to the Close Date regardless of the Stage, you will need to create a custom formula field to calculate those days.