The ages of 16 to 18 are a "gray area" for any types of sexual relations. But overall, if the 16 year old has not been emancipated, any sexual relations are subject to criminal prosecutions. Consent may be a defense, depending upon the specifics of the relationship. But overall, this remains a dangerous relationship. While dating is not against the law, sexual relations may be. Should the 16 year old's parents seek law enforcement's help, it could spell great trouble for the 20 year old, if accusations fly. Perhaps the best course would be for the 16 and 20 year old to sit and speak with the minor's parents to define the relationship and sexual parameters.
This is definitely a dangerous situation for the 20 y/o. Dating is legal, but most other acts frequently involved with dating are not. I would suggest even heavy petting could be troublesome. Absent a healthy relationship with the 16 y/o's parents and family, trouble could easily arise. Unless this is a serious relationship and true love exists, I would advise the 20 y/o to lay off at least until the 16 y/o reaches 18. If that is not going to happen, then I advise they do not engage in any sexual activity until the 16 y/o reaches 18. There is a possibility of prosecution if the 16 y/o becomes pregnant, even if there is no report of illegal sexual activity to authorities. If law enforcement finds out about the pregnancy, they could investigate. I admit, in my experience, this rarely happens, but you could be the first case.
I apologize for the incorrect answer I gave. I didn't pay attention to the fact the girl was 16, not fifteen. Yes, the age of consent in Florida is 16 and sexual relations in this case should not be criminalized.