LOL, I definitely talk with DS4 as if he is older. For the most part as I do other adults, other than the fact that I still use discretion in things that I talk about at timeS. I don't talk with him about a murder I saw on the news or anything since he is not even 5yo. I think when they act the way they do and talk the way they do, it's only natural. When DS was 3 and in a prek program, the teacher said to me "I reason with him and talk with him like I do my 8 year old" I can remember times where people didn't quite know he was the way he was and at 2 or before would speak to him in sing song voices and say things like "can you say nose...where is your nose?" and he would stare at them as if there was something wrong with them. Sometimes he would just go along with it. Despite the fact I talk with him more mature, I still play things up like you should for kids at times. Like he will make me a bracelet and pick flowers and things and I will be overly enthusiastic. It makes him really happy and makes me smile. My general nature is pretty laid back so it wouldn't be good to just shrug off things like that. : )
And the bubble gum tree answers and other things like that drive me nuts. If I don't know things we look it up. Although I do recall a time a while ago when I was overwhelmed with all the questions and he was asking about how doorbells work and I said "you press the button and it rings" and he continued to want more information, LOL. But we did look it up and I didn't say anything dumb about a doorbell fairy or anything.
With DD2, we talk pretty mature to her too. We are just starting to have conversations with her so it's a little different of course. But I do talk with her as I talk with other 2yo's.
