Interesting thread this.
A close relative of mine had dementia. When it came to the point there was no recognition when I visited, I would start singing a song from her younger days. She would join in and sing it right through, word perfect in perfect pitch without me. It never ceased to amaze me. I have had similar experiences with other elderly people who are basically non-communative due to dementia or aging problems.
As far as songs go, we probably all have our own favourite 'decade' for sure however, most of us know various songs that have been popular over the last century. A couple of years ago, I heard a teenager singing "We'll Meet Again". It did (pleasantly) surprise me, I must admit.
Music is a powerful tool in all aspects of life.