I can't speak to what happened in France, but I can assure you that in America, what you're describing has been a hobby of older generations for easily a century and a half. There are even at least two words in the English language that I'm aware of that came about because a particular group of adults was overly concerned with the reading and/or academic abilities of a younger generation.

The first term, adolescence, emerged as a term to describe the period between childhood and adulthood in the early 20th century and was, in part, shaped by a belief by those who practiced a new field of psychology known as child study that there was something different about the generation of young people they were seeing in schools and their families. While not all practitioners saw what young people did as disordered or wrong, they were as a general consensus that the world had changed so much that there was a need to understand the modern (as it were) child. I get more into the history of that movement in a question on asking children, "what's your favorite color?"

The second term is drop-out and emerged in the 1960s to describe a young person who did not finish high school. Prior to the post-World War II "Baby Boom", a young person could leave formal education without a formal degree without any major social or economic negative consequences. There were some regional differences, but generally speaking, a young man, especially a white, non-disabled young man, without a degree could get a well-paying job without the academic knowledge that a high school diploma signified. As norms shifted, the term emerged as a way to describe a young person who lacked, or was otherwise, uninterested in academic pursuits, which is to say reading.

If we go back in history, there are college professors from the late 1800s complaining about the terrible penmanship they were seeing from incoming students. To the early 1800s where professors at the Colonial Colleges routinely complained about the poor Latin and Greek skills among their students. Which is to say, adults have always found a reason to complain about the academic skills of younger people.

To focus, though, more specifically on reading, the difference in reading between different generations in the latter half of the 20th century is less about quantity and more about the nature of what young people were reading. Between 1950 and 2000, several new genres emerged explicitly for young people. These include the "YA" books (Nancy Drew, The Hardy Boys, Sweet Valley High, etc.) then middle readers (Goosebumps, Captain Underpants, etc.), not to mention Harry Potter, and post 2000 series like Twlight, Hunger Games, etc. Then there's fanfiction and blogging and message boards etc. Young people pre-2000 read a lot. We'll leave the reading habits of young people post-2000 for a future thread.

Boomers, which is to say those born after World War II, experienced some of the youth-oriented publishing but not in the same way as their children and their children did. So, to a certain extent, what the adults are likely speaking to is that the nature of things put in front of young people's eyeballs, or under their fingertips or in their ears, has changed fairly dramatically since they were young people. If you're curious, I get into some of the history related to the rise of periodicals for young people in this question on the history of raising white children to be enslavers. If you're interested in the evolution of what young people read in American English class, I get into that here, here, and here.