Summary

1)  Warning-ng  marks messages as new although they are outstanding in some cases.

2)  When it happens more then once it also might mess the age information.

It seems that adding git forensics might help to improve the detection algorithm. Some thoughts about what can be done will be added as a comment.

The problem 

As you can see the message at line 9264 was marked as new therefore it's age was reset to 1.

but all lines were committed at the same time (only part of the line is shown) and were not changed in the last years. 

Moreover every build that the detection fails gets new referenceId for that message and as a result the Age of the warning is reset to 1. 

As a result after some builds with changes on other lines the age of lines that were committed together might be totally messed up as can be seen below. 

you can see that for the same commit we get different ages. 

The reason is that those messages were identified (incorrectly) as new along the time and their age was reset to 1 at the time.

see the 502_diff.xlsx attachment for more details