It's not quite what you were asking for, but since I'm not familiar with any 55+ condo communities in Cambridge, I wanted to share a link with you that you may find helpful:
http://www.chapa.org/pdf/AgeRestrictedHousinginMA.pdf. This is an 81-page PDF, quite long to read but quite a bit of information inside.
55+ are marketed as a "lifestyle", however I, in my cynical nature, feel that this is a way to market older properties and not have to worry about issues such as lead paint, etc., which may be found in the older buildings that are the majority of housing within the Boston/Cambridge area. Once you get outside of the city itself, you may find 55+ communities that have on-site pool, gyms, and other amenities.