It is not illegal for a 15 year-old to date an 18 year old, but it is illegal for them to have sexual contact of any kind, and that includes kissing.

Can a 16 and 18 year old legally date in Florida?

In Florida, the age of consent is 18 years old, sexual intercourse with someone who is under 18 is considered statutory rape. There is a close-in-age exemption (Romeo & Juliet Law) allowing minors who are 16 or 17 to have sex with someone no older than 23 years old.