the duration between found the defect and rectified defect 
is measured as defect age.

The time gap between the found the issues  and fix issues 
is called defect age

The time period of found defect to fix the defect is called 
defect age.

Defect age is time between defect identified Vs defect
injected. (Defect injected in Req Phase and its been
detected in Testing Phase)so the defect age would be Testing
Phase - Requirements Phase.

Defect age is a measure of the duration for which a defect 
remains in the product or in other words the defect age is 
a measure of the duration between defect detection and  
defect injection. Ideally the defect age should be zero, 
which means that the defect is removed as soon as it is 
injected.