They will have problems everywhere in the USA at their age. The age of majority there is 21, no exceptions. If their trip is cancellable with no penalty, I would recommend that they leave it until the are 21. I went to the USA on a work visa with official USA documentation and proof of age. It was still a complete pain every time I checked in anywhere, or wanted to visit a bar.

No its not possible to cancel now
They have at least 2 options in Vegas where they accept under 21's
I have just emailed the hotel in LA where they were wanting to stay to make sure there is no
problem with them.
There are places where students can stay (like when they take a year out of Uni and travel about, there are places that accept under 21's) but they did want something a bit better than a hostel type place.
They want some adventure while they are both still at uni and have no real responsibilities
They aren't bothered for bars/drinking etc so wont be attempting those.
They have gig tickets for Vegas to see a band and that is an all ages gig in the Palms.

I think the Luxor accepts 18 year olds checking into a room. However if the MGM accepts them I would stick with that as it's a good hotel and in a far better location than the other two.

luci