Sorting by the highest Age Grade percentage is another common use case of custom sorts. Here's how you would set that up:
1). Build a New Report (or edit or duplicate an existing Report)
2). Add an Overall Section.
3). Open up the "Add Columns" Panel and add "Age Grade" as a column to your Report.
3). Add "Age Grade" as the Sort Override in the "Filters & Sorts" Panel.
4). Uncheck "Ascending" so that the highest Age Grades are at the top of the report.
That gives you something like this:
You can publish your new Age Grade Report like any other to RunSignup as explained here: Publishing Online Results to RunSignUp : RaceDay Scoring (rdscoring.com)