Before 2018, online and retail sports betting were only allowed in Nevada. Since then, legislation has been passed in 30 states legalizing sports betting.
There is no one legal betting age in the US, as there is for drinking alcohol. Instead, each state has individual laws, and they vary based on the type of gambling. In many states, the betting age is 18 for daily fantasy sports (DFS), lottery, horse racing, and sweepstakes casinos. The gambling age for online sports betting, retail sports betting, and online casino is typically 21, although there are exceptions.
Since you don’t have to be a resident in the state that legalized gambling, standing in its borders is enough. It's important to know the laws and regulations for each state just to be safe.