Old folks seem to relate best to songs from their teen years or early 20's (i.e., the 'courting' years). That means if the clients are 85 years old, that songs from the 30's would be spot on, and they would easily remember earlier one which were 'golden oldies' to them at the time.

A good play is name songs, especially the common names, for example:

I Wonder What's Become of Sally
Oh, Johnny! Oh, Johnny!
Mary's a Grand Old Name
Darling Nelly Gray
Big Bad Bill From Louisville
I'm Just Wild About Harry
I'll Take You Home Again Kathleen
Danny Boy
Lydia, The Tattooed Lady
Ragtime Cowboy Joe
Sweet Marie
Sweet Rosie O'Grady
If You Knew Suzie Like I Know Suzie
Molly Malone
When You and I Were Young, Maggie

This way, you can focus on a member of the audience, put him/her in the spotlight, get everyone involved. And, they already know the songs, too.