Seriously, what's wrong with being 5'3"? Does this mean that, by default because of my height, I'm a stupid moron? I'd take offense at that, except that I'm not 5'3". I am close though. I'm also much older than 12 (much older... Less than 30 though, by a fair amount). Height and age have very little correlation, in my opinion. Though, this is in part because I've been fairly short much of my life...
Anyway, if someone tells you their age over the internet, take it with a grain of salt. Sometimes even a handful. I mean, I could say I'm a 12 year old girl (that'd be disturbing...), but that doesn't mean I am... To quote a comic I found funny: "Over the internet, no one knows your a dog." (as side note, *please* note that "no" and "one" is two words. Noon is noon, not no one.)
If someone isn't acting the age they said they are, then there's a very good chance the information they gave you is incorrect. This might also hold true, if they're acting *too* much their age...
Yah, anyway, I'll stop now. Mouser X out.