HUD (federal) defines housing for older persons as that which is intended for occupancy by at least one person in each unit to be at least 55 years old. In order to protect a surviving spouse or cohabitant that is younger than 55, federal law only requires 80% of the occupants to meet the minimum age requirement. Example: A couple aged 57 and 52 purchase a unit within an adult community. Shortly after their move-in, the 57 year old dies. Without a provision for some occupants to be under the age of 55, the surviving 52 year old would be forced to sell and move.
HUD (federal) law does not prohibit a complex from selling up to 20% of itâ€™s units to those under age 55. Some complexes will not, because the complex uses their allocation to protect surviving spouses or co-habitants. Other complexes may market a percentage of their units to buyers who are under age 55.
The Housing for Older Persons Act (HOPA) provides for adult communities to be exempt from Fair Housing Laws which prohibit discrimination based upon familial status. HOPA defines the minimum requirement for a community. State law, or even a specific complex/development may maintain more restrictive requirements.
Here is a link for more details on the Housing for Older Persons Act. You can check with the Offices for Aging in PA for any state laws which may govern senior housing.