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.
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.