As someone who has been reading story books since she could read, I find that most age reccomendations like middle school or young adult are just plain annoying.
I like to read novels with mature themes, not smut but just mature themes, and it's so annoying that I constantly have to explain to people that just because a book is rated A or R doesn't mean that it has smut.
Additonally, targetted age groups are obviously meant for the average audience. A teenager who reads mature themes like crime, psychopathy, manipulation, power play, etc. will be more comfortable reading a novel regarding the same rather than a middle aged person who rarely even thinks about these even if the novel is not marketed as young adult.
What a person can handle heavily varies from person to person and while I see the point of age based targetting/deeming whether or not something is appropriate for an entire age group, I personally think that more teenagers and children would benefit from reading about things that aren't specifically marketed for them. If it's happening in the world, they can and should know about it.
Teenagers and Children literally have the free time that it takes to explore and attempt to understand the world, it's what they should be doing, then what is the point of shielding them from certain facts of the world.
Also, just because a book has a sexual scene or is about sex doesn't mean that it's porn. For example: Eleven Minutes by Paulo Coelho. I personally believe that the vast majority of people should read it before they have sexual encounters.
Another point is that these labels inherently don't make sense because young adult/middle school mean that they are targetted for that specific age grp while adult literally just means everything that is not dumbed down specifically for children.
As someone who is in their late teens this is a difficult system to explain to people who buy me books, aka my parents.
It would be far better if we divided it into kids and everyone else, people would be free to choose and those who don't roam the book store as a hobby won't confuse movie and book 'adult' labels.
In short, age is a not a good meter of judging a person's reading level after say, the age of 7, and hence, should not be used for the same. Instead we should all just have books marked for graphic content/potential triggers.