I have only passed threw Bellagio, usually I stay at the Four Seasons because I dont want to get into the whole your money is worthless until your 21. So I choose to stay at FS because they are one of the few properties on the strip that will take my money using FHR. I have stayed there so much that now it would be pointless asking me for ID, I guess unless I cash a check. I would imagine Bellagio would ask for ID, but it could be to verify your name. Normally at hotels where I think that the check-in age might be 21 or higher, I just give my passport because they really don't want to sit there counting your age. Any other type of ID would be your best bet because its not highlighted with "UNDER 21 UNTIL" I really think it depends on who checks you in and at what time because they might be really busy and not have time to check anything.