Joe, first off, I'm so proud of you for what you are doing.
What I do, to establish a good musical reference point, is to ask their year of HS graduation. Sort of an offhanded way of asking their age. That year is most vivid in everyone's memory. Then, knowing the year, go to
www.tropicalglen.com and find the Top 100 Pop Hits for that year. Alternately, the Top 20 Countdown. They also have the Top 100 Country Hits. They have every year between 1950 and 1989. That should satisfy most elderly listeners. They have reached an age where they are proud to admit it publicly.
All the best to you on your commendable project.