Updated July 26, 2023
This is one of those questions for which the answer is, it depends on whom you ask.
The 21-or-older rule applies to all Caesars Entertainment and MGM Resorts hotels on the Strip, even non-gaming hotels such as Vdara and Delano. However, you can stay at CityCenter, where Waldorf Astoria will allow 18-, 19, and 20-year olds to book a room; of course, you must show valid ID and have a credit card and money should be no object.
A few other Strip hotels cater to 18-year-olds, namely Elara and The Four Seasons, both of which are non-gaming properties.
As for locals casinos, all Station Casinos and Boyd Gaming require a minimum age of 21.
Much to our surprise, a few casino-hotels allow 18- to 20-year-olds to book a room. Westgae and Tuscany have an 18-and-up policy. The Golden Nugget has a two-tier policy, allowing 18-year-olds to rent regular rooms, but it's 21 or older for a suite or luxury room.
Boutique hotel Rumor entertains 19- and 20-year-olds if they’re from out of state, but visiting Nevadans must be at least 21.
The most ecumenical policy is found at Residence Inn Convention Center, which rents to anyone with a credit card and valid ID. In fact, the Convention Center area is an excellent one in which to stay if you’re 18 and in Vegas, because of a high concentration of hotel rooms you can book.
Aside from the ones already mentioned, Las Vegas hotels with an 18-year-old threshold of entry are:
Alexis Park
Americas Best Value Inn Downtown
Best Western Plus (Henderson)
Courtyard Las Vegas – Convention Center
Extended Stay Hotel Flamingo
Fairfield Inn & Suites Airport
Hilton Grand Vacation Suites Convention Center
Hilton Grand Vacation Suites Flamingo
Hilton Lake Las Vegas Resort
La Quinta Inn – McCarran
Marriott Grand Chateau
Renaissance Las Vegas
Royal Resort
Springhill Suites (must have own credit card)