Based on these criteria, people suggesting old movies and novels are barking completely up the wrong tree.
A person under 25 can easily pass as human if they have never seen any movie made before they were 5 and never seen any movie that wasn’t in the top 3 most popular movies for their age group the year it was released.
If a 20yo has never seen any movie made before 2005, chances are nobody will ever find out and nobody will care of they do find out. The same will be true if they haven’t seen two of the three Hobbit movies or never saw “Gravity” That’s just not that unusual and not important for pop-culture literacy.
So really, the number of movies will be maybe 10 or fifteen. Only children’s movies need to have been made more than 5 years ago. Not having seen recent kids movies like Frozen or the Lego Movie wont be unusual, especially for males. So really you only need to compile a list of the top 3 movies most popular with teens made since 2010. That’s 15 movies. Then add the most popular kids movie for each year between 2000 and 2010, another 10 movies. That will be more than adequate to ensure cultural literacy for a 20yo. In fact you could probably halve that number.
The whole internet thing is mostly a teen phenomenon of the past 7-8 years. It’s going to be crucially important for a 20yo to be intimately familiar with how to use Facetube and YouBlog or whatever. That means they will need to spend at least a few weeks just posting and reading comments and playing with the systems to familiarise themselves with both the software and the social dynamics. This can not be taught in class.
But by far the biggest will be TV and popular songs. A “normal” 20yo will have been exposed to almost a quarter of a million hours of TV. It’s going to be hard to pass as normal without an intimate familiarity with all the kids programming from the past 15 years. And that means all the characters from infants shows like Sesame Street and Yo Gabba Gabba, Pokemeon, at least a majority of the plot developments of major kids shows like The Simpsons, Power Rangers and so forth. Then you need at least working familiarity with shows like Bear Grylls, Oprah (still within the consciousness of a 20yo), TMZ and a plethora of reality TV shows. And thanks to re-runs you will also need familiarity with Friends, Everybody Loves Raymond and a host of other shows that well predate a 20yo.
There are going to be over a hundred shows that a 20yo will need to be intimately familiar with to pass successfully and another 500 that they will need to have pasing familiarity with the setting, characters etc. When someone makes a joke that a guy looks like some character from Yo Gabba Gabba or Freinds, they need to be able to recognise that almost instantly to pass.
Music will be much the same. The amount of music that a person of that age has absorbed by osmosis is vast. It will basically need to include every top 20 song of the past 10 years as well as a sizable selection of popular songs from previous eras.
Reading material is really optional. It’s perfectly reasonable for a 20yo not to have read any specific novel. Even really popular kids books like Harry Potter only managed about 20% market saturation at their peak, which was before this person’s time. So really, you can skip the reading material altogether.
In term so older culture, I think a lot of people are overestimating how literate the average 20yo is in that regard. If a 20yo didn’t know who Charlie Chaplin or Marilyn Monroe was, that wouldn’t ring anybody’s alarm bells. I would expect that over half of 20yos wouldn’t know both of them. More recent big name stars are probably more relevant but even there, if they haven’t appeared in a major movie in the past 10 years nobody is going to be surprised if a 20yo doesn’t recognise them.