At what age do u go up a level in nipple size on bottles?

Bottle nipple size. The answer is different for every baby, depending on how big s/he is and how fast s/he is growing. Usually it's pretty obvious that the baby is working very hard at sucking to get out the increase volume of milk that growing babies need. If the baby is sucking comfortably and emptying the bottle in the usual amount of time s/he takes, there is no need to change.