This assumes there's a correlation between karma/age and the tendency to promote fairly (i.e. due to civility and positive contribution, rather than promoting what one agrees with).

I doubt there is. The problem is that no one judges the judges. That is, you don't lose karma points by making bad judgment calls. The same problem exists at my work. People become managers by being good engineers and by having been there a long time, but no one is judging their management skills.