It's a problem of speed of changes, old people can't have a true discussion to young people anymore.
I've noticed that whenever I've been to France. I learned French in school, and I notice there's an age cut-off when I can no longer understand what people are saying.
A small consolation is that the people who no longer speak intelligible French are usually somewhat better at English.
This is nothing unique to French, it is just the generational evolution of language. Listen to a news report in any language from the 1920s. It is vastly different in tone, style, vocabulary and possibly accent than a news report today. Change is the only constant.