Knowingly owning or using classified info without proper clearance is, in fact, a crime.
That’s a large part of what Trump’s classified document raid was for. Former presidents usually have a lot of classified stuff to turn over after leaving office. It’s standard practice, (and perfectly legal) to simply send it back (via the proper channels) as soon as you discover you have it. But if you conceal it and refuse to return it (like Trump did) then that’ll land you in some hot water.