Currently in the flagging section of the moderator tools a user's join date is highlighted in varying colors based on how far in the past it is relative to today's date. Here's an example:
This is useful as an indicator that the flag is potentially more likely to be valid as the user is new to the site. Unfortunately though it becomes useless when older posts are flagged even though the flagged post was created very soon after the user joined, like here:
Here the flagged answer was posted the same minute the user joined, and they were thus a very new user at the time they posted answer. Yet it's not nearly as obvious at first glance that this is the case as it is above.
As is discussed here, just because a user has been around for a while and has potentially accumulated a good amount of reputation doesn't mean their old post that is flagged should be treated any differently:
Those same high rep users may well have been low rep users when they posted that answer, and could well be embarrassed by it if they remembered it, so do them a favor and help get rid of it.
After all, we were all new users at one point and were potentially far more unaware of the FAQ, etc., then than we hopefully are now.
That said, I think it makes far more sense for the join date to be highlighted based on its proximity to the flagged post's date rather than the current date so that it better reflects how new the user was at the time they wrote the post rather than how long they've been a user in total.