I think Marcus just wanted to be cool and brag about his stay in Vegas.
Yes, tweeting about your adventures in Vegas doesn't necessarily mean it will stay in Vegas, not this time (or ever for that matter).
He does look older than 21 though, so I guess he could get free drinks and gamble and not be questioned about it.
Even at the age of 21, I had the looks of a teenager. And was carded quite often for buying alcohol up to the age of 30.
Good luck trying to weasel out of this one Marcus...live and learn.