DD2 will be 4 in a few months. She still sleeps happily in her cot (which is one of the bigger ones with a 140cm mattress) and we have kept the bars up. She has never really complained or tried to climb, although she is tall and perfectly capable of doing it. She can be very difficult at bed time, will fight sleep and keep herself entertained singing or playing with her cuddly toys, but still happy to be inside the cot really. Is it time to switch to a bed rail? Have we left it for too long?