To be honest I never had any sense of talking to air, through the ages of growing up. Needless to say, I don't think I had the mind to even think of such a thing. The only time I'd ever done that was while sleep walking, and damn well scared my mom lol.
Regardless, ages 3-7 tops would be my best guess.
I'm no expert, but I would think that if a child has an imaginary friend, it would be ok until the child reaches about the age of nine or so. After that I wouldn't consider it to be normal for a child of that age to have an imaginiary friend.
1) There are two sides to this - you could argue that it provides a basis for communication skills and a sense of creativity from a young age, or that it can cause social deprivation, preventing a child from making 'real' friends becasue they feel they do not need them. Personally, I think it's a good thing, but not to the point where they completely lock themselves away in their own imagination and do not interact with the outside world at all.
2) I never did when I was younger, but when I was about 13 I started drawing as a proper hobby, and created my own characters for stories and comics. Since then these characters seem to have taken on a life of their own and exist inside my head almost as separate people to me. It's not multiple personality disorder or anything like that, but I'm a very introverted person who dislikes socialising (Asperger's Syndrome) so I think it's my way of dealing with being lonely. I also have a very vivid imagination, so it surprises me that I never had any imaginary companions as a young child.
3) I predicted that most people would say around 7-10 years old. A lot of people agree - I read an article a few weeks back that imaginary friends in teenagers and adults could be an indicator of schizophrenia or multiple personality disorder. However it doesn't necessarily make you 'weird', if they exist just inside your head. I'm 16, and I have fictional characters that live inside my head. Does that make me a freak?
1) I tend to side with the creative development argument. However, if it becomes some sort of manic obsession, then parents should quite possibly become worried.
2) My imagination has always been fairly vivid and I also wound up turning to art (well, music and dance for me) as an outlet. I'm actually surprised to hear you have Asperger's because you always seemed outgoing to me. Oh well, you're just as awesome either way.
3) I think anyone who is highly creative will always have some kind of character living in their head. Whether it's people from their comics (yeah Latios, you know Hyper lives in your head. ), or a stage persona, or some other form of the same thing, all creative minds will have some kind of character inside of them.
2) The way I present myself over the Internet is completely different to the way I am in real life. I have a problem with face-to-face interaction and physically being around other people, so this is a lot easier for me.
3) True, very true. A lot of people I've met on deviantART have talked about imagining their characters in their head as a development exercise, but I doubt there are many that do it quite so vividly or frequently as I do. I don' care if people call me a freak, it works for me and it keeps me happy.
3) Firstly, call you a freak, and they'll die. Sorry, couldn't resist. But seriously, you have to develop your characters/personas somehow. If you can't imagine it in your head, you probably can't do it. Like I said, some amount of imagining it is probably a requirement to good character development.