maybe a dumb question

I partially agree with BarbaraOK, but I do not believe they get any tax advantages. It is against the law to discriminate on age. The government has set up two scenarios that will keep a senior community legal. The first is that all residents must be 55+, we'll call that the restricted version. Each community is allowed to then set visiting restrictions, like family over 21 can live there, grandkids can visit for 2 weeks and no longer, etc. Some of these will allow a younger spouse with restrictions if one is 55+. The second more flexible version says that it is primarily a 55+ community but not exclusively. I have seen the number 80% in the past. They also are allowed to make up additional rules. We had a case in Cape Coral, FL where the wife was 55+, the husband was younger and they had a teenager, probably a grandchild. Everything was OK as long as the wife was alive. She up and died and the rest of the family was required to leave the condo even though they couldn't sell it and had no money to buy somewhere else. Of course they were still responsible for the Condo fees on the condo they could not live in
I believe you are seeing the 55+ because they have a lot of permanent residents and they choose to follow one of those two scenarios. The only way to know the difference is to ask what their rules are. I would not be afraid that you won't be able to find a place to stay.