We sleep trained our 3year old when she was 13 months and thank god we did. Our 10 month old used to be a good sleeper and has turned into a horror of late. She has no idea how to put herself to sleep. We want to sleep train her as well, but she sleeps right next door to our 3 year old and we don't want our 3 year old to be disturbed. We used CIO with our 3 year old.