Can a fifteen year old date an eighteen year old?
There is no law about dating ages, however, there are very specific laws about doing more than holding hands. In most states, any sexual contact would be criminal conduct.
And why would an 18-year-old want to date a 15 year old? There is only one thing they are going to be after and it isn't scintellating conversation.
Excuse me I have no extra info for this answer but I'm deeply in love with a girl relativly 3 years younger than me, and we're both pledged until after marrige virgins who wear purity rings every day all the time. I don't know who kicked you in the butt, but you need to take a few steps back and take another look!
PS The highlight of my life is dating her, and just dating her so there!