As mentioned, the legal gambling age will vary across different states in the USA. Each state is its own jurisdictions and, while federal laws apply, gambling is a strict state affair. Therefore, lawmakers are free to seek consensus on the gambling age.
To gamble for real money in the United States today you have to be either 18 or 21. However, there are some specific exemptions for tribal casinos. While commercial casinos may only admit you if you are 21, a tribal casino may allow you to enter on 18, because they enjoy sovereignty.
Presently, there are 35 states where you can only gamble if you are 21+ and 22 states that allow 18+ individuals to enter and gamble.