Training children

So I'm soliciting opinions on something I've been thinking about lately. The topic is training children in such things as sitting still (Catherine, your recent milestone there with Hannah got me thinking more about this) and not destroying something that other (older) kids have made while playing on the floor and can range to such things as potty training. I see two schools of thought - 1, you can't possibly expect a child of x age to be able to do y (ex. not knock down something that his older sibling has made) so don't even try to correct that behavior because as someone at a party said to me when I was trying to tell Dylan to stop messing up this puzzle I was helping and another child with, "you're expecting him to not act his age" - 2, kids should learn right from wrong as early as possible and children learn by repetition so you need to start earlier than later. And for things like potty training or signing, what does it hurt if you start early (as long as you are willing to deal with the frustration until they are "ready.") I think what complicates this is that, as we hear so often, kids are so different developmentally and so it's hard to tell someone or may be looking for answers to this question when their child is "ready" to be potty trained or sit still, etc. Obviously there's no easy answer to this and it depends a lot on the specific instance (potty training is a lot different than teaching a kid not to touch something hot) but I'm wondering if you all have any insights as you have thought about this issue.

To the grandmother who told me that I was expecting Dylan to not act his age, I wish I would have said "I don't expect him to obey at this point, but I still want him to ingrain in him what is acceptable behavior." I'm not sure the grandmother would have disagreed with me. I think her point might have been, which I totally agree with, is that too often parents get frustrated when kids don't do what they are told even after many repetitions, not realizing that kids just aren't wired like us to be able to do that. It takes much training and much repetition (and a certain unknown age?) until a child will behave. That frustration can lead to things like abuse if taken to the extreme but that doesn't mean we shouldn't keep telling our kids that something is wrong even if they don't understand it yet.