For example, I want to add a filter that filters a map by age group: children, adults, seniors. The distribution of people in these groups is very different and I would like the legend on the dashboard to change based on which filter is applied.
Hi Kimberly Goswiller,
The Legend Element inherits properties from the Web Map's Legend: Map legend—Operations Dashboard for ArcGIS | Documentation
Currently, this legend does not change based on the filter applied in the Web Map. An Idea has been logged for this functionality here: https://community.esri.com/ideas/13817
Hope this helps,