Can a 15 year old date a 18 year old?

Having sex with him ( what of course is on the mind of many 18 year olds) may be illegal depending on where you live.

But for dating, as long as your parents approve and give their permission to date a 18 year old, there is no problem dating

Fr_Chuck is right on the money. If your parents are not on board with this, they could possibly (depending on where you live and what the age of consent is in your state) call the police and have him arrested for statutory rape, which doesn't have to even involve sex. But if they're okay with it, then all should be well.

But seriously, an 18-year-old is going to have far different expectations from the average 15-year-old. I don't know how experienced you are in the dating area, but developmentally, you're much closer to childhood than he is. And I would guess that he'll be leaving the area (or at least graduating from school and going to work) very soon, which gives this relationship a limited life expectancy. You might be better off looking at some of the boys closer to your own age for now. Just my opinion, but after 25 years of teaching high school, I've got lots of experience to base it on.