Last Activity date fields, which only available in Report, List View, and API (including Formula Field, Workflow and Validation Rule) for Account, Contact, Opportunity and Lead object.
This blog will cover 2 more fields : Age and Stage Duration. These fields only available in Standard Opportunity report type, but not using custom opportunity report type.
For open opportunity, the number of days since the opportunity Created Date until Today.
For closed opportunity, the number of days between opportunity Created Date until Close Date (not when it closed). This include Closed Won and Closed Lost opportunities.
The is number of days of opportunity from the date when opportunity change to current Stage until Today. The number of days still counting even for closed opportunity. You can monitor Last Modified date in Stage History of opportunity related list .