Somebody described the problem a year ago here:
http://jenkins.361315.n4.nabble.com/Problem-with-Age-column-on-Test-Results-tab-td3172208.html

I have a problem with 'Age' column on 'Test Results' tab. For couple of my tests, all the time this column has value equals '1', despite the fact that those tests start failing earlier than one build ago. When I switch to 'History' tab, in 'Test Result' column there is a 'Regression' value for all builds, and it should be 'Regression' value only for the first build and 'Failed' for next builds.

For me this happens because I have many junit xmls that containing the same test suite name.
In this case hudson.tasks.junit.CaseResult.getPreviousResult() gets the only last junit xml result and if it's not failed then the Age column won't be calculated properly.