Legally, no.
The same age restrictions apply to gambling as to drinking: 21 is the minimum age and the casino risks a big fine if minors are allowed to loiter on the gaming floor.
The resorts are laid out in such a way that it's virtually impossible to get from A to B without passing through a gaming area. Hence, it's okay for someone underage to walk through as long as he or she is accompanied by an adult over 21 and clearly in transit.
Of course, the closer you are (or look) to 21, the better your chances of getting away with hanging around in the casino, or even gambling. Casino personnel -- including dealers when you walk up to a table -- will ask for ID if they're suspicious, but they don't check everyone. But if you gamble and win and turn out to be under age, you won't get paid, so why bother trying?
Although there's plenty to do with the kids, Las Vegas is essentially an adult playground and many hotels won't even allow you to book a room if you're not at least 21. The minimum drinking age is as rigorously enforced as it is for gambling, if not more so. It's common practice for people in their 30s to get carded. Don't expect to get into bars or clubs without valid ID.
An exception to this is if the venue has a clearly demarcated eating area, separate from the bar, and you're there to eat.
Other exceptions include the Flirt Lounge at the Rio, which admits all Chippendales ticket holders (the show has an age restriction of 18 and older), and some concerts at the Joint at the Hard Rock and Mandalay Bay's House of Blues, which are open to all ages (check with the venue before you purchase tickets, to see if an age restriction applies).
Last but not least, if you want a truly adult night out, all-nude strip clubs that don't serve alcohol will admit patrons 18+.