My DD was 18months but i do have friends who made the move earlier (14-16months), like others said its up to the child and how they are. My DD had no transition issues i put her "big girls bed" in a new room that had been decorated for her and gave her the choice one night. I just asked if she wanted her cot or big girls bed and she went and stood beside the bed, we've never looked back and i shut the door at night (she cant yet open it) this saves her from roaming the house as i know she would get up to mischief if she got out. Will you be using a bassinette for your new bub? if so maybe you wont need to hurry her into a big bed.