My view is that it's my house and if they actually did clean it properly and put things away every so often, then I'd deal with some clutter. But the reality is that their idea of "cleaning" means all clothes from the floor goes into the hamper which makes work for ME! And most everything else gets stuffed into the closet or under the bed.
Perhaps you can still "work" with your 6 yr old and devise some simple reward system for keeping his room clean. (Also I've found for my kids that 90% of the mess is due to too much stuff in their rooms, lack of storage they can reach well and "old" stuff like out of season clothes/too small clothes/lots of books, etc. Get the clutter out of his room, buy him some new storage and he might be inclined to put everything in it's place.)
As for your pre-teen, maybe he has a point? He wants to have some privacy and boundaries - then why not encourage that...Mommy style! If he wants to deal with his room, clothes and personal affects, tell him you're not helping him - when he's lost something, doing his own laundry, vacuum and dust, etc. Let him get a taste of "adulthood". Let him find his lost library book. Let him dig out that missing soccer cleat. And finally, start HIM on doing his own laundry. Maybe giving him what he's asking for will help him to appreciate all that you do for him. And IF he comes crawling back to you, asking for you to "help" him do his laundry, then he needs to keep his room immaculate, except for maybe his closet.
The bottom line is, I'm in to "giving my kids what they ask for"...and then enjoy watching them squirm when they seen all it really entails. Then I get a compliant child who doesn't want to do all the work, but will happily follow my requests, without any screaming or nagging.
BTW - my kids are young grade schoolers. So for them (like your 6 yr old), I give them time to clean up their rooms. When it gets really bad, they are "stuck" in their rooms (like grounding) for that day, until it's cleaned. If they argue or put up a big stink, I come in with a laundry basket and CLEAN the room - all toys, clothes, etc. on the floor become MINE and they have to earn them back.
They now know that I'm serious when I tell them to clean up. I've explained that this is OUR HOUSE and that no room should be a "pit" of dirty laundry and unable to be vacuumed.
So whenever they want a taste of responsibility, I'm happy to comply...Mommy-style!