A book becomes a classic when it is read by lots of people for a long time. That time span may be decades long, but it's often a century or more.
As a writer I can tell you a lot of the classic books wouldn't have a chance of getting accepted by a publishing house today. Writers are told we must start in the middle of the action and avoid a lot of description, but older books often spend pages setting scenes before the stories actually start. I guess in those days people knew less about environments other than the places they lived in, so they really needed a lot of information to set the scenes.
However classics are classics because they're good reads and people still enjoy many of them today.
And lots of classics written for kids are still popular with children.
In my local library some books that were originally intended for adults are found in the Children's Section. They don't contain any material that's offensive, except for some racist and sexist stereotypes that were the norm back when they were written. Kids can learn a lot about how our culture used to be by reading classic books.
I'm currently reading The Complete Science Fiction Treasury of H. G. Wells. The anthology contains seven novels and is 760 pages long, so I may not finish it before next week. But his novels are certainly worth reading. I'm amazed at the creative mind that man had!
Have you read any classic books lately? If not, you might try some.