In Marin county, most of the active adult community has an age limit of 55 and above. If it's a couple, one of the persons will need to be at least 55 years old to qualify.
Check with the community you are interested in for their specific requirements, make sure the amenities, services, activities and surrounding environment are what you are looking for.
M aunt and her husband, although much older than 55 are definitely the 'Very Young 70s'., both in heart and physical condition. They moved to an active adult community in CO. This is near the university so they can participate in many university sponsored classes and activities. They are thoroughly enjoying the experience.