Rachel, there is no "right" age. For us, we moved DS1 into a bed at around 16 or 17 months because we had our second baby on the way and we didn't want two cots. Also he seemed ready, he was sleeping well and also sleeping on a toddler bed at day care with no problems. The transition went really well and he continued to sleep well in the bed. DS2 is still in a cot at 18 months, but we will probably convert it into a bed pretty soon. I think that while they fit in the cot, there is no need to move them until they are close to climbing out (For safety reasons). But it also ok to do it earlier if they seem ready.