DS (4) is better at putting things back after himself than DD (9). He's well trained at nursery I think.
Rules are they need to tidy toys away, plates by the sink, dirty clothes in washing baskets then room tidied (I give DS a helping hand as his room isn't very organised atm. When he gets his new room he'll be 5 and I will expect him to keep that tidy (will make sure he has lots of storage and a place for everything to make it easier)
I am trying to get DD to get into the habit of tidying her room every evening for a few minutes rather than it getting bad and seeming too much to do in one go. I help her when I feel she needs a bit of help.