Do a search of “55 plus” on the forum. You’ll find some good info. I believe Frank at one time suggested contacting HUD or Fair Housing. You may also want to try the local licensing/zoning person. If it’s officially a 55+ park they may be able to tell you.
I’m doing the same research for a 55+ community I’m considering making an offer on. I didn’t have any luck with HUD or the Fair Housing people unfortunately. As it turns out, they likely can’t tell me because they don’t have a record of it being one. After doing some digging I learned that one of the previous owners only allowed 55+ people to live there and it isn’t actually, in the eyes of the authorities, a 55+ community. I’ve found this to be true with a few communities that are ‘55 +’.
Try asking the owner if it actually is 55+. They may tell you that it really isn’t.