Gaming regulations allow minors to pass through gaming areas, but not to loiter. If someone under 21 is loitering near the tables or gaming machines then security would politely ask them to move along.
Unfortunately, you must be at least 21 years old to gamble in Las Vegas. The legal gambling age of 21 also applies to the rest of Nevada, so you won't be able to try your luck elsewhere in the state.
All in all, there are free slots that pay real money, but you will have to be playing at real money online casinos as opposed to social casinos on slot apps, or playing titles in demo mode.
Yes, some free slot machine apps offer the chance to win real money. These apps often have in-app purchases or advertisements that generate revenue for the developers, and they might also have a system where you can earn virtual currency or tokens by playing the free slot games.