Florida meets all of the criteria. With that being said, and not knowing the age of your parents, IMHO this state isn't the best place for older people to vacation or live. Everything is expensive, ppl are hostile and the drivers are horrible. Almost all vacation type activites revolve around nightlife something I doubt your parents would be into. Only three real reasons to go to the Keys. Fish, SCUBA or get drunk. Don't get me wrong my father used to have a place in Marathon and I spent a lot of time down there but for my father, who was in the late 50s age group, there wasn't much to do other than fish.
Have they considered somewhere overseas? Mexico, Costa Rica or Belize(like a previous poster recommended)? I know people who take cruises to Belize and Mexico and they love it.