When our club plays for seniors (and I hasten to note that many of us are seniors!) we measure success by how many people sing along. It depends on age and to some extent geography (folk, country/western, jazz standards and show tunes don't all play equally well with all audiences).
Many of today's retirees were screaming from the balcony when The Beatles played Ed Sullivan. So don't feel compelled to stick with pre-rock songs. We get great response with:
The Great American Songbook: Blue Skies, Pennies from Heaven, Shine on Harvest Moon... songs everybody knows, especially from Irving Berlin and Harold Arlen
40's swing for the oldest folks: Sentimental Journey, Chattanooga Choo-Choo, Boogie-Woogie Bugle Boy
50's rock: Chuck Berry, Elvis and Buddy Holly
60's: The Beatles, Motown, Herman's Hermits! (Something Tells Me I'm Into Something Good, I'm Henery the Eighth I Am)
Del Shannon: Runaway. 100% participation on "whyyy-yyy-yyy-yyy-yy she went away."
John Denver: Country Roads. Everybody sings, including the folks celebrating their 100th birthdays
Bobby McFerrin: Don't Worry, Be Happy. Nearly everybody knows it, and the others enjoy it anyway.
Hawaiian standards because you play ukulele and people expect it: Tiny Bubbles, Ukulele Lady and -- yes indeed -- Let's Talk Dirty in Hawaiian. The guys at the VA hospital loved that.
All that said, the real thing is for you to enjoy playing the song. If you have fun, the audience will, too.
ETA: Ack, I forgot the #1 all-time best song! It is, and brace yourself: That's Amore! Schmaltz it up and everyone will have a blast.