The OP has a legitimate complaint.

With a normal guaranteed Marriott.com reservation -- with absolutely no wording about any age restriction -- the OP had a legitimate expectation that the reservation would be honored.

Marriott.com reservations provide details about pet policy, smoking policy, parking, and all sort of other factors which vary (or don't vary) from one hotel to another. But reservations for the Cosmopolitan have nothing about something as significant as an age requirement?

Marriott's Ultimate Reservation Guarantee protect guests against overbooking situations, but the wording is not limited to overbooking situations.

I'm sure the OP was inconvenienced. In fact, the OP was probably inconvenienced quite a bit, given the size of the Cosmopolitan and the process checking in at a mega-resort (and then having to leave again). The Cosmopolitan owes the OP SOMETHING. And, considering that Marriott has a published policy guaranteeing reservations, that would seem to be the perfect solution.