I agree with everyone else. Their house, their rules. If you say anything, your step-son may not take offense, but you better know his girlfriend WILL (and think about the future if this girlfriend becomes his wife, then you may have already ruined the DIL/MIL relationship--don't do anything but praise their choices really! The 'future DIL' will remember everything you guys say!). For people that age, a big screen tv is furniture, ha ha. Again, their money, their choices, etc. I'm pretty sure they are busy doing (HMMM) other things since they just moved in. Wink wink.

If you 'own the house' they are renting or buying, etc., then you still don't have a say really unless they do 'damage' to the place. Clutter doesn't equal damage unless of course it's to the point of bugs, rodents, etc. I suppose. Truly though, if they haven't learned how to 'take care of themselves' in their mid/late twenties, then it's a little late now (and I know you said you have never lived with either so I'm not directing that at you OP). Let them 'live and learn'. They'll get it together.