Very mixed results.

One guy absolutely nailed lap style guitar basics very fast and went on to play anything that interested him happily, though he didn't live long after that.

Another guy made repeated excuses of how he was incapable of learning new guitar things, then proved himself wrong. What he seemed incapable of was practicing.

One woman understood the function of the mandolin, learned basic sight reading but refused to get an instrument that was playable. Instead, she insisted on struggling with an almost unplayable piece of crap. Nothing would persuade her otherwise. She quit on account of it.

Another woman, on ukulele, could figure things be ear okay but couldn't grasp any visual representation of music. She also couldn't remember chords at all.

One of my current retirees, learned sight reading on the mandolin, chords, and plays beautifully. He struggles a little with some keys as his fingers don't always react to new fingerings quickly, but he knows when he's made a mistake and he fixes them.

It's a tricky and challenging proposition to teach the elderly, but worth the effort. Everyone, regardless of age or infirmity, deserves a chance.

Peace, Mooh.