My nephew is now 3.65 years old, and I'm amazed at how abstract his thinking has become in the last few months. Tonight he said "That makes her nervous." I forget who he was talking about -- it doesn't matter. I can't believe he knows the word "nervous," or knows the proper way to use it, a fairly complicated emotional notion.
You can talk to him about almost everything, and hold a conversation, and have a good time with him for at least a half-hour at a time. Eight month ago he would ask lots of questions like "What does that mean? when you used the word "ketchup," but anymore there's none of that. He has lots of funny things to say, too, and he knows it. He is so simple and alive. His growth is astronomical.
On the other hand, he is fairly obsessed with poop right now, and it comes up ever 7 minutes or so.