My girls are 2.5 and just about 4 and I don't think either one understands what this means. Obviously my 2 year old is quite young, but I'm wondering if there's an easy way to explain it to my oldest without scaring her. Our fish died a few weeks back and I said "he died" and I'll often say things like "oh that plant died" or "it's ok the bug is dead"...but I'm not sure they really get it. Obviously those are very minor examples (thank goodness).

Is there an age where kids should start to really understand this type of thing? How do you explain it?