Mate, to be honest, I can understand, I mean I am now 21, not that old, but when I go out to pubs and clubs and I see 18-17-16 year old girls, I feel old because, since I started working almost 2 years ago, I've grown up a hell of a lot and I see people that age are pretty immature, and yeah, I'd agree a bit scary and excitable. I mean my gf now is 18 and she is a bit like that, but I like that, makes me feel a bit younger, but was a bit weird to get used to.
I wouldn't worry too much, I mean your only 18, as they say in Road Trip "you'll have to pay for chicks like this when your 40" maybe you look a bit younger, not 18 and that's why you don;t seam to attract 18 year old girls, as people have said women tend to go for people a bit older (note: not the rule of thumb). Saying this, my gf's mum was 16 and her dad was 27 when they met and their still married now haha