I did it at 28 months and it worked fine. I think the sometimes it's ok, sometimes not is difficult for them to understand. At that age you can explain it to her. One day I told DD that beginning that night we wouldn't nurse after bedtime, then I reminded her when we went to bed that there would be no more nursing until the sun was up. When she awoke, I refused nursing, offered water and cuddling and was understanding of her feelings about it. On the fifth night, I think, she awoke but didn't ask to nurse and put herself back to sleep. After that she started sleeping all night. We continued cosleeping throughout.