Reading comprehension isn’t really tested as a thing once you’re out of full time education, unless you have a learning disability. We test it explicitly for children up to 16, looking for it to match their chronological age, then to measure learning disabilities.

Sometimes instructions are tested for readability/ reading age. The US Navy did a lot of this work, originally. It needed “uneducated” enlisted men to be able to read the handbooks!

As an English graduate, and English teacher, I can reassure you, the difference is taste. I read Joyce’s Ulysses as a teen and didn’t notice any difficulties. I cannot get past Ch 3 of Tom Jones. It’s not difficulty; it’s taste.

Plan a presentation for your book club. See if you can show them the charm of Gravity’s Rainbow