Lots of sites have this kind of thing - and the commonest implementation I see is "five whatevers" (eg five stars or five coffee beans or in our case five paperclips?) where they start out grey and progressively get coloured-in to indicate... not time-since joined but a combination of that and of active participation in the community (usually numbers of posts and replies).

We could easily compare time-joined to karma points. EY et al would get five paperclips, a newbie with no karma would start with none. The paperclips could work on a logarithmic scale.