Rigorously.
Regardless of past PR attempts to market Las Vegas as a family destination, it remains, and always has, essentially an adult playground, where much of the entertainment is geared toward those 21 and older. For example, many hotels won't even allow you to book a room if you're not at least 21. Of course, for those bringing children, there are plenty of rides, shows, pools, arcades, and other attractions to keep the kids entertained, but much of what this city has to offer is strictly adults-only.
The minimum drinking age in Las Vegas is 21 and it’s as strict at the minimum gambling age (also 21). It’s common practice for people in their 30s and even young-looking 40s to get carded. Never expect to get into bars or clubs without valid ID. And the ID has to be valid; even the best Internet knockoffs can be picked off by bouncers who look at government identification cards all night long.
One 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 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 admit patrons 18 and older.