Indian casinos that run anything beyond electronic bingo have to enter into a compact with the state where their reservation's within. Typically, the deal is that they get to operate slots and maybe tables in exchange for a percentage of profits or a lump sum going to the state, as a sort of "tax", since Natives living on Tribal land aren't subject to income tax.
However, patrons at Indian casinos are subject to income tax, and all wins over $1200 are documented and the winners get themselves a nice n' shiny new W2G.
As for the age thing, it depends on the tribe's compact.