I think the main reason older people don't like younger peoples' music is because over time you start to forget just how terrible most music for a particular era is. The crap of today is more readily apparent than the crap of the past.

Give this a try: Listen to the radio-top-100 songs of a particular decade and see how much of it is any good. 80% or more of it is utter crap. Its only possible redeeming quality would be the triggering of some nostalgic episode of your past which nobody else outside of your generation shares.

I have favorite songs going all the way back to the 17th century, but rarely are there more than 20 per decade (or 20 per century, going beyond 1920 or so), because most of the stuff was crap.

I think old people often lack context, which is part of what this post is saying but I think the post doesn't quite capture an important part of what I mean. Unfortunately, I am not sure I can explain myself adequately.