For me it used to be 18 months when I move them to cots (since this is how daycare centers here do it when kids change rooms). I just started doing it earlier with my current crew (around 13 months) since I had an illness and one of the babies was to heavy for me to hold him and get him in the playpen. I spoke with all parents before making the switch and all agreed. Now I have two 14 mos old and one 13 mo sleeping in cots. I transitioned them to cots almost at the same time I was transitioning them to daycare. They stay in the cot. I had one of them that got out once and try to wander around, but after my stern voice telling him not to do it he stopped it.