I started pretty much from birth. There are no precious things ground level. So by the time he was walking, he was too small to reach the mantlepiece so that wasn't a fuss. When he could, and when he could climb to reach the bookshelf in the sitting room, he was able to understand that not everything gets thrown.

BUT he was also a bit of an attached child (his choice!) so he could see these things. If he asked to see them, I would pick it up and remind him that it wasn't a toy before letting him touch. Like knives: he can help me cut things ONLY when I know it's safe. We have safe knives that won't hurt him, but I still do knife safety every time as sometimes he does use sharper objects (such as the apple wedger) that he needs to be careful with.