DS is 22 months and we've not felt the need to use a naughty step yet, I don't think he's old enough to be intentionally 'naughty'. If he does anything he shouldn't, e.g. recently he's realised he can climb onto the coffee table and we told him 'no' because its not very stable and might fall over, which seemed to work, although he has done it again since and we just keep saying no each time and he gets off.
But I was at a friend's the other day and her DS who is 2 climbed onto a table and was taken straight to the naughty step. She said she'd been using the step since he was one year old.
It just made me think that seemed very young - or am I being too relaxed and DS will end up thinking he can get away with anything??