Las Vegas casinos are allowed to have both topless pools and topless shows -- provided they follow all local laws and ordinances, of which there are too many to enumerate.
Casinos located outside Las Vegas city limits have different -- and generally stricter -- regulations. When Green Valley Ranch in Henderson announced that their small adult-oriented pool (The Pond) would be top-optional, the property was informed by local law enforcement that topless pools are not allowed in Henderson. These days M Resort, also in Henderson, gets around this regulation at their locals and party-oriented Daydream Pool Club by allowing women to be topless, provided they don pasties (available at no extra charge from the pool staff).
One early topless-pool experiment involved Sapphire Gentleman's Club taking over one of the pools at the Rio; it was short-lived, due to a whirl of arrests for drug and prostitution offenses. Later, when Sapphire was granted permission to be the first gentlemen's club with a swimming pool all of its own, it had to agree to keep things squeaky clean and abide by a variation of the hotel-pool regulations that outlaw female toplessness if the venue is hosting a party (ironic, because every day that Sapphire Dayclub is operating is a party).
Another toptional experiment centered around the GO pool at the Flamingo. But that whole concept turned out to be in direct contravention of the law in Clark County, which states that: "A resort-hotel may provide for its guests a specially designated portion of its swimming pool area where topless sunbathing is permitted. However, such a specially designated area must be separated from all other swimming pool and guest areas; be obstructed from the view of patrons in other swimming pool and common areas; be off-limits to all minors under the age of eighteen; and cannot be used for any special events, contests or parties while any topless sunbathing is taking place."
While the GO Pool conformed to all other criteria, it most certainly did not comply when it came to special events, and the media hype its raucous parties quickly attracted soon spelled the kiss of death to any toptionality.
Hence, in order to stay in compliance, Tao Beach at Venetian allows topless females on sedate weekdays, but never on weekends, when the celebrity hotspot dissolves into a seething mass of party animals.
Red Rock Resort had a topless pool a few years ago (Cherry Cabana Club) but the demise of this venue stemmed from a lack of interest from female guests. If a pool is just an "adult" (no minors) pool that’s not topless, then the hotel no longer has to worry about whether or not their pool scene is a party, or suitably obscured from outside view etc., so if there's no or only a minimal market for bearing it all, it's simply not worth the bother to accommodate the odd daring soul/exhibitionist/European.
All the adult pools in Las Vegas, whether topless or not, are restricted to guests over 21; they all serve alcoholic beverages in the pool areas. As explained above, topless pools must also be located outside the view of both the general public and of hotel guests who are not specifically at the pool.
As for completely nude sunbathing, it is not allowed at casino/hotel properties in Nevada, period.
When it comes to topless casino shows and erotic revues, there are age restrictions for the performers, crew, and audience, based on the availability of alcoholic beverages in the facility.
Topless shows may allow guests under 21 if the show is in a theater with no beverage service during the show, but topless shows cannot allow minors to attend (under 18).
Totally nude gentlemen's clubs may also permit those aged 18 and up -- both on and off stage -- because these establishments are not permitted to serve alcohol, with the exception of the Palomino, which does serve liquor and not only has all-nude female entertainers, but also the only legal all-nude male strippers in the U.S., on account of the club pre-dating the laws that outlaw its policies.
To conclude, here's our guide to Las Vegas topless laws in a nutshell. You’ve got to be 18 to see nipples in this town, but you can see breasts at 16 as long as the nipples are covered.
You’ve got to be 21 to see breasts in a pool environment. But if it’s a pool party, then no one can see nipples, no matter how old you (or they) are. Nipples and pool parties just don’t mix.
You can’t be or see completely naked at a pool, even if it’s not a party, but you can see naked and be naked in a show, any age from 18 up, provided there’s no alcohol served; if booze is served, then nipples are okay, but not full nudity.
You can’t have alcoholic beverages served in a room in Las Vegas where there are female genitalia on display; however, if you go to North Las Vegas, Palomino Club and Lacy’s Lounge have fully nude shows <i>and</i> full bar service.
(For in-depth coverage of gentlemen's club rules and etiquette, check out ToplessVegasOnline, which addresses everything from the letter of the law when it comes to touching, safe-parking tips, privacy issues, how to avoid being hustled, the best clubs, and much much more.)