The elementary kids, so we get everyone up in time to eat and read scriptures before the HS kids leave for seminary.
Usually getting the 2-10 year olds down is tougher than the MS/HS age kids (who have to stay up for homework, etc), so an earlier start means an earlier bed time. Put them down before they are tired and stalling, and let the older ones do like soupie said.
Also, decide on a time you want to be done and just go to bed whether clean-up is done or not. If you don't get it done, it'll still be there tomorrow.