Complex sentence: Editors who have been around for a while and have shown they know how to write or improve articles, and have learned how to implement the various policies sometimes become administrators-but they have also demonstrated that they act sensibly and in a grown up manner.
Split into simpler sentences: Editors who have shown they know how to write or improve articles, and have learned how to implement the various policies sometimes become administrators. They have lots of experience and have also demonstrated that they act sensibly and in a grown up manner.