It depends on the child and the upbringing. I think given the opportunity children can grasp these things earlier than previously thought. My kids started to show signs of this from about 12 months, in small things and its only progressed from there. Now at 3 DS is extremely aware of logic and reason and I found due to the fact we've always explained everything it has prevented tantrums or stopped them. I think if you allow them to have the opportunity to learn reason and logic they will. But often people automatically assume a child/toddler/infant couldn't possibly understand so they don't communicate this way with their children. Everyone used to be flabbergasted that DS understood things so well at such a young age, a gf of mine (who had 3 older kids) adopted our some of our communicative skills and found that her son benefited greatly.

My advice is talk to you kids, when they start to get it you'll know, but don't automatically assume they don't understand
:yeahthat:

DS has had good comprehension, reason & logic from very early on and we've always put it down to explaining things to him, whether we think he'll understand or not.. but obviously he did! hehe.

DD has understood for a while now different warnings, such as careful of your fingers when you close the drawer.. you can see her little mind tick over and she flattens her hand out as she shuts the drawer.. or when she's in the bath I say to be careful when she's standing up because she'll hit her head on the tap.. she'll look up at the tap, stand up and point at the tap. So she knows exactly what I'm talking about and why.