I have checked into my room in a hotel on the Strip when I was 20 with my Amex. No problems whatsoever. I was aked for ID (as were all people checking in at the time) and presented a Drivers License with the red UNDER 21 banner on it. They didnt care. As far as settling the room bill your money is still green and your card is still Platinum.

However, there is no reason that your associate cannot check in on your behalf. I have had to do that on numerous occasion where I missed a flight and the rest of my party got there before me. They will present their card to cover incidentals. At any time you can swap cards on file from his to yours to allow you to sign charges to the room on your card.

On another note I also had no problems drinking and gaming but I cannot advise you on doing either of those.