There are no laws regarding dating. There are laws about sexual contact. And since the only reason an 18 year old would date a 15 year old is illegal, it isn't going to work.
Well it all depends really. If you really like the person, go for it! And if the girl is younger, it may be more apropriate then it its the guy who is younger. Its really all on your own judgment :)
Normally, the legal age of consent is at age 16. There are no laws against dating but there are laws in against engaging in sexual intercourse with a minor. Though the usual age difference is up to 3...