I saw there was a similar request few years ago (Badge suggestion: Badge for being active member for 3 or 5 years) and I didn't know how to bring that forward again.

I think is just natural extension to have a gold badge that rewards long time commitment much like the silver Yearling badge does.

Lustrum badge will be given to people that have over 5 consecutive Yearling badges and have an overall score over 3k (or 5k or 10k the best amount could be decided here). In any case the score should be higher than the 1k that is necessary for the 5 consecutive Yearling badges.

The badge could look like this:

I went for the word "lustrum" since I think it sounds good (at least better than semidecadian or quinquennium) and represent 5 years in one word.

Also this could be awarded multiple times much like Yearling, thus in 2018 there could be people that have two Lustrum badges.

To a suggestion in the comments this question was copied as an answer here