It varies widely. Some places have limits on number of days per year that anyone under the age limit can stay, while others are more liberal and only require that at least one member of the household is over the age limit. However, in most of them, proposed "underage" permanent residents require a review and permission by the HOA, because they are set up to take advantage of certain tax benefits, and if a set percentage of residents are not retirement age, they will lose that. The usual rule is that the more "active adult" focused the place is, the more structured activities there are, the stricter the rules tend to be. Rental property also plays into this; the more rentals the place allows, the stricter the rules tend to be, to be sure that the age distribution does not creep down.
Many 55+ communities will allow prospective residents to come for a trial visit by renting a place for several weeks, even if rentals are not their usual market. A good real estate agent can advise you.
Pinellas county, in particular, has a wide variety of places; they went after the 55+ market starting in the 1920's. The variety of places in Pinellas covers nearly every price range and style of residence, from high-rise city apartments to gated golf communities. (And if you are thinking your 20-something kids will live with you part-time, proximity to St. Pete will probably be important to them.)
PS: While most people know that Florida has no state income tax, many people do not realize that it is also a homestead exemption state, which means that part of the value of your home is exempt from property tax. (Only applies to full-time residents with no second home in another state). In some cases a 100% exemption applies, if you qualify.
https://floridarevenue.com/property/pages/Taxpayers_Exemptions.aspx