Certain characteristics can be selected for ("all female" is legal) but it is of doubtful legality to post discriminatory housing ads in terms of age.

This link[1] agrees with you, but I'm wondering how retirement communities get around this. I'm not talking about assisted living, but places like this[2].

[1] http://www.nolo.com/legal-encyclopedia/free-books/renters-ri...

[2] https://en.wikipedia.org/wiki/Laguna_Woods_Village

The Housing for Older Persons Act of 1995 http://www.manufacturedhousing.org/publications/default.asp?...

This is for the landlord-tenant relationship. My understanding is that it's legal for people to advertise for roommates (with whom they share living space) based on pretty much whatever they want.