The Time Interval Between Defect found and defect closure is called defect age.
Once any defect is reported by tester , It will be assigned to the developer then developer will work on fixing of defect , after fixing the defect again QA needs to verify whether it is working fine or not. if is failed it will be reopened and developer need to fix if defect is passed QA will close that defect. This Time interval is called as Defect-age
Formula:
Defect age= Defect closed date -Defect identified date
For example if the defect is identified on 12-Sep-2015 10:00 AM and closed on 30-Sep-2015 10:00 AM then defect age would be
Defect age= (30-Sep-2015 10:00 AM) -( 12-Sep-2015 10:00 AM)
= 18 Days
Defect age= Defect closed date -Defect identified date
For example if the defect is identified on 12-Sep-2015 10:00 AM and closed on 30-Sep-2015 10:00 AM then defect age would be
Defect age= (30-Sep-2015 10:00 AM) -( 12-Sep-2015 10:00 AM)
= 18 Days