Usually done on defects that have not been sent back to QA as fixed. Shows how long defects have been open and also the number that have been open for the same period of time. I usually do this by Defect Severity as we expect the turnaround on high level defects to be faster that those on low level defects.
It depends on your environment. In some cases, the acceptable age of a defect varies based on priority. In many cases, we allow low priority defects to remain open far longer, and then have a period of 'liquidation' where the focus of the developer is simply fixing as many bugs as possible - and this would be when they'd be more likely to get low bugs fixed.
If you're having a problem specifically with bugs of varying priority, it sometimes helps to quantify what is acceptable and communicate that on to the development manager so that teams can work together to see things addressed.