Hi Lindsay,
This is a great question! At my previous institution, we counted emeritus volunteers as those who gave more than 500 hours over their volunteering career. We gave those volunteers a year's family membership to the museum, a small box with a plaque on top with the volunteers name/years volunteered, and an invitation recognition to that year's awards ceremony. I thought the box was a little kitschy, but our older volunteers seemed to like it. If a volunteer had more than 1,000 hours over their volunteer time, we continued to invite them to the volunteer social events. You can obviously tailor the hour amount requirement to your museum.
Most of the volunteers who reached the higher level emeritus status didn't continue to come to events; most of them retired for health reasons. I liked being able to give them a family membership because even if they weren't interested in visiting the museum themselves any more, they were allowed to put it in someone's name who would use it. Most often this was a son/daughter/neighbor with kids.
I do thing emeritus volunteer recognition is something we needed to analyze and decide what would be the most thoughtful and valuable way to recognize our volunteers who contributed a lot. Sometimes it was hard to tell- someone would "retire" for two years for health or personal reasons, and then jump back in with us because their health problems were resolved or we were able to reengage them.
Do you have anything in place currently?
Samantha