Understanding the concept of a "living thing" is a late developmental achievement. Early research by Jean Piaget, showed that kids attribute "life status" to things that move on their own (e.g. clouds or bikes) and even 10-year-olds have difficulty understanding the scope of a living thing.
Wow. That has some big ramifications to me. To be honest, though, it doesn't surprise me. First of all, if it takes humans this long, it is obvious that this is a concept animals would struggle with. And what follows to me is the interesting tie-in to the 'Age of Accountability' discussion. The question is, if you still don't truly understand what it means to be alive, do you understand what to be human really is? And then, do you understand that you have a soul?