In Florida it is legal for someone 18 yrs old to own and possess a handgun as per F.S. 790.25. They can be 18 and buy in a face to face sale from a non dealer. To buy from a dealer, they must be 21 and go through the 3 day wait. No-one without an FFl or by going through an FFL may buy interstate handguns.
Various sites can be checked to verify all;. The BATFE site is pretty good at spelling it out. You would need to search your own states statutes for legal ownership of a handgun.