"a space he envisions as a dorm for Millennials" - he's pretty much over the line from the start if that's an accurate representation of his goals.

IINM, at least in the US, such laws don't apply to "shared dwelling spaces", which these would count as. (Not a lawyer.)

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.

Yeah, the fact you have to audition for an overpriced room is kind of offputting, too.