I live just outside Gainesville. There are definitely some younger (50's) retirees here. But this is the youngest county in the state b/c of the university. There is so much to do here that is free or very low cost, though. Outdoor recreation is very popular: canoing the rivers and springs, hiking state parks, golf, water skiing, etc.
Really, though, it's difficult to give you a good place to live in the state without knowing what you're interested in. Do you want to be in or very close to a big city; prefer the Gulf or the Atlantic; prefer the beach or the lake/river lifestyle; want a mini-ranch with a few horses? This state is incredibly diverse. If you let us know more about what you want out of your place, I'm sure we can point you in the right direction.