Well you are different, one of the side effects of sexual reproduction - nothing wrong with having confidence in yourself, it can be a healthy form of self delusion, healthier than deluding yorself that you're worthless for the most part - you can be "special", without neccessarily being "better", in a social sense, just different - everyone has unique individual talents that they should develop and learn to share.
Anyway, I believe schizophrenia usually manifests in the late twenties, early thirties, but there is often a brief symptomatic phase in earlier years - it may indicate problems later on - don't worry about it, but be aware of it. It's a condition similar in ways to borderline, which is really an entirely different thing - nathan schwartz-salant has written an excellent book on the subject, "the borderline personality: vision and healing" you might find useful.
As I explained to nawtaprincess below, these voices are essentially powerless, disconcerting and disturbing maybe, but they can't really hurt you - the most reasonable and least complicated explanation is that neurons firing randomly are triggering random cognitive/memory symbols in your subconscious/unconscious, that most people ordinarily have no access to except in dreams, etc. - they are "irrational" in the sense that they make no sense in any conscious way, but represent deeper symbolic processes that are difficult to analyze empirically as they seem to bear no relation to anything on the cognitive level.
This is scary, it's unorganized (in the conscious sense), and difficult to discuss with anyone - two reasons why schizophrenia sufferer's often become paranoid, imo - it's a "bad trip", very emotionally stressfull, and diffcult to find anyone to talk to about, while the disorganized nature of the experience demands some effort at putting it into an organized context - thus, elaborate and highly logical theories are formed, the conscious minds attempt to organize the sub/unconscious symbols leaking across the limnal barrier, influenced by the accompanying emotional stress.
Poeple in this condition are also not always *completely* delusional, and it can be a very creative state - nietzsche springs to mind, but at soem point it's just too much stimulus, and can lead to either oversimplification (fascissistic) or evercomlexification (paranoia).
Overstimulation can also lead to catatonia, there is just too much information bleeding across the barrier to keep track of or make sense of, and cognitive processes can be overwhelmed or shut down.
Trust your senses: if there isn't anybody actually at your door, they're probobly not out to get you, and the most harmful aspect of it is probobly the stress itself which can have physiological consequences on your health - you need to reduce it, and this is the best reason to get some professional help, someone with some experience who isn't going to be totally at sea with it.