Defect age is the time elapsed between the day the tester discovered a defect and the day the developer got it fixed.
While estimating the age of a defect, consider the following points:
The day of birth of a defect is the day it got assigned and accepted by the development team.
The issues which got dropped are out of the scope.
Age can be both in hours or days.
The end time is the day the defect got verified and closed, not just the day it got fixed by the development team.