My theory is that there are two causes.
The first is hearing.
Like it or not, I am sure that this does become less sharp as we age, if only slightly if you are lucky.
The second is about the natural changes that take place in how people speak over time.
When you are young, you naturally absorb all the speaking-styles that you are exposed to. As you age, your ability to do this lessens.
On top of that, you probably have less exposure to the 'latest' ways of speaking (as well as to other forms of English, such as American), so again this reduces your ability to process them.
In other words, the older you get, the harder it becomes to understand how younger people speak.
As I said, it's just my personal theory, and I'm sure bit won't take long for it to be blown out of the water. ☺
Actually, I more often find that I can't tell what is being said in very old films, from the 'fifties or earlier, and I think that is for almost exactly the same reasons - the speaking style is one that I wasn't exposed to as a child (born in the early 'sixties), so my brain can't process it all.
I wonder if others of my generation have the same problem, while the older Gransnetters, who grew up watching those films, still understand them ok?