Does the waiver of age and service really say on "or before"? Or does it say anyone working on the Effective Date enters on the Effective Date?
You can't ever ignore service before the Effective Date so to me the answer here hangs on the exact wording of the waiver.
If they really wrote people employed on or before the Effective Date they were sloppy in writing the document. If they meant to write "or before" and didn't think of rehires they were still sloppy.
I would think about getting the attorney who wrote this document (if there was one or the person who wrote it) to opine on this.
Once you set precedent I would document so the plan is consistent going forward. I would also get the client to sign off as Plan Administrator to agree.
However, if it really has the "or before" language in place I would tend to agree with Riley- or before means or before.