Umm, . . . it is a major no-no. They will get snippy about it.
If he is at a table, he'll likely be carded; he may be carded if he's just in the casino.
He might get away with playing the slots some, . . but if he were to win big, the casino would not give him the winnings. It is against Nevada State Law for the casino to pay an underage winner.
If the Gaming Commission Of Nevada Agents find an underage person gambling, the Commission will fine that gambling establishment heavily.
The fines for a single offense can be in the hundreds-of-thousands-of-dollars.
Some years ago, when Harrah's was still dealing decent blackjack games DonDiego was playing near a table where a young fellow had not been carded, and the on-site gaming agent asked to see his ID. He was underage.
The game was stopped immediately. The dealer was immediately removed and cited for not carding the kid. The pit-boss was also removed. DonDiego doesn't remember the details, but one person was fired and another received a suspension. The dealers and pit-critters spread the story like wildfire through every pit in the casino.
So, an underage gambler can not only cause himself grief, but he can get casino employees in serious trouble.