It depends. Most 55+ communities allow residents below 55 if the homeowner is 55+ years old and is actually living there. Not all elderly people can live alone, so it makes sense to allow their kids or caretakers to live with them.
According to the Fair Housing Act, to be considered a 55+ community, it has to have people over 55 living in at least 80% of its units. That means the maximum amount of residents below 55 years old you can have is 20% of the units/homes.