Housing for Older Persons Act of 1995 (HOPA).
80/20 means they must have 80% at least 1 person of 55+ living in each residence. The remaining 20% is discretionary and can be zero % under 55. Annually updated By-laws for each community will dictate their allowances. The 20% usually is used as a buffer, allowances for inheritances due to deaths and things like that. If you are renting in one and under 55, the Home Owners Association can tell you to leave, after your lease expires. If an owner is renting to under 55 and then in the future the HOA suddenly doesn't like you, those "Condo Commandos" could find something in their by-laws that can be used for force you out. Other laws may prohibit or delay such actions... get an HOPA Lawyer for more info!