I've talked to myself since I was four or five years old - I've ALWAYS talked out loud and thought out loud; never a time when I didn't. I've been embarrassed a few time when I've argued with myself a bit too loudly and folks caught me. :/
When I was a kid, I used to pretend I was being interviewed for some TV show or movie gig, or I was giving a talk to a group of people. Seriously, I'd never heard of TED talks until maybe two years ago, but I used to pretend I was doing that sort of crap at eight years old. Had some really interesting and deep, philosophical ideas on the merits of cartoon cats, and why they were misaligned in society (seriously. I made up a whole lecture series on this. At ten years old. I was a WEIRD kid).
I attribute all of this to me being an only child in a single parent household, though. I had a fairly active imagination and I entertained myself a lot.