There are no criminal statutes in Florida that make it illegal for a 17 year old and a 21 year old to engage in consensual sexual activity together.
For you and your friend's knowledge (please spread it around), there are two sets of numbers for young people in Florida to keep in mind to stay on the right side of the law if they are engaging in sexual activity.
The first number is 16. For practical purposes, it is never legal for anyone in Florida to have sex (or engage in "sexual activity") with another person under 16. That behavior is criminalized (at various levels of severity based on the differences in age) in the Florida Statutes at section 800.04: http://www.leg.state.fl.us/Statutes/index.cfm?App_mode=Display_Statute&Search_String=&URL=Ch0800/Sec04.HTM.
The second set of numbers to keep in mind is 16,17 and 24. A person 24 years old (or older) cannot legally have sexual activity with a person 16, or 17 years old. The statute dealing with that is Section 794.05: http://www.leg.state.fl.us/Statutes/index.cfm?App_mode=Display_Statute&Search_String=&URL=Ch0794/Sec05.HTM
Together these statutes mean that Florida has two "ages of consent." 16 years old is the age of consent that applies to EVERYONE's partner, no matter how young the older person is (for example, if a 14 year old and a 15 year old have sex, they are BOTH committing a crime). But, for a person 24 years old or older, that person's partner must be 18 years old to give consent to sexual activity that will protect the older person from being charged criminally.
Excellent answer Atty. Berns. I like the way you explained the various statutes. I am posting not as an answer but as a follow up question.
Does the Florida legislature realize how oxymoronic it is to authorize the prosecution of either individual if both of the participants are under 16? If the rationale for criminalizing sex with someone under 16 is that they are legally deemed to be incapable of consent and the purpose of the statute is to protect the underaged person from being exploited, how can the same person be both a victim and an offender?
Are there any appellate cases in Florida dealing with the situation that it is most likely only males who would be prosecuted in the case of two underaged people having sex? Mass. had a case about discovery in the prosecution of a 14 year old boy who was charged with various kinds of sexual activity with younger girls. The court ruled that the defense was entitled to information to attempt to show that the statutory rape statute was discriminatorily applied.