I try to delay it as long as I can because it is one less thing to worry about at naptime. If possible I wait till the child has moved to a bed at home. Sometimes it has to happen because I need the room they are in for a new younger child. To transition I move their playpen to the nap room and they sleep in it till they show me that they can fall asleep , stay asleep and stay laying down till I come back. Then they move to a mat but they are reminded using a mat like the big kids is a privelege that if they abuse they will lose and have to go back into the playpen. Most of my kids go to a mat at age 2 but as I said size and what is going on at home matter.