DS1 moved to a bed at around 16 months due to the impending arrival of DS2. The move went really smoothly, but I know others who have had problems at this age too, so it definitely depends on the child. Preparation is also a key - we got Jack used to it ahead of time, talking up the "big boy bed" and doing the bedtime routine on his bed for a few nights before he actually slept in it. It worked a charm.