I've read that they absolutely must be out of the crib by the time their armpits are level with the top rail. Our pediatrician recommended moving my twins when they were 18 months, and was pretty convincing when I heard about all of the head injuries he had treated over the years. You may think your toddler has never tried to climb out, but you can't be there 100% of the time, and you may not be there the one time he does try and is successful. By the way, moving them at 18 months was SUPER EASY because they were in a very easy-going stage, and didn't know they had the option to refuse. Once kids reach the terrible twos, where they say "no" to everything, it's much harder. Also, our Dr. explained that the 4 walls of their room become the walls of their crib, and so every thing in the room has to be secure, and you must be sure they can't get out on their own.