This first link is to the official HOPA (Housing for Older Persons) Act. It is not exactly light reading. The second link is a FAQ page which is much more easy to read and should answer most, if not all, questions.
http://www.hud.gov/offices/fheo/library/hopa_final.pdf
http://www.hud.gov/offices/fheo/library/hopa95.pdf
Note that the 80%, one person 55+ requirement is the absolute minimum to comply with federal law and maintain HOPA status. If a community fails to maintain and properly document their compliance, then the community would lose their HOPA status. However, individual communities can have much more stringent policies if they so desire. For instance, they could require that 100% of Homes be occupied by a 55+, or 60+, etc.