The age difference between a 15 year old and an 18 year old isn't large. It's only a three year difference. They could easily be in love. If they aren't in love than that's a different story but if they are, it's completely and entirely acceptable. When they are much older like 45 and 48. The difference won't seem like a lot and it probably won't matter.
In today's society, most 15-year-olds are as mentally prepared to date as 18-year-olds are. I do not see a difference in a 17-year-old dating a 15-year-old than an 18-year-old dating a younger person. Even though 18 is considered an adult by many standards, 21 is much closer to adulthood. Some people are more mature than others, and having a set rule for dating ages is not conducive for everyone.
Children as young as 15 should not be allowed to go out adults as old as 18. A 15 year old does not have enough maturity to make that kind of decision. All children should wait until their 18th birthday to date. You're a lot more adult decisions involved in dating.