In my experience with DD1 and DD2, the earlier you start with demonstrating correct behaviours the better. I say demonstrate because initially they listen to you and they do understand but don't necessarily copy. But at least you are showng her what she needs to be doing so that when she is old enough to copy consistently and to understand the concept of manners, she knows exactly what is expected of her because that's all she's ever been shown iykwim? My DD1 started to understand a bit more at about 18 months but didn't speak properly or understand the concept of manners until about two. But I still persevered with it. I'm doing the same with DD2 now but she's a fair way off understanding properly yet. You'll know when she's understanding you a bit more. I think when their language develops it's much easier to teach manners. Hope that made sense!