Indeed, it's a state-level law in the US, and less than a quarter of the states mandate 18. For everyone else, it's -lower-.Which highlights how dumb these rules are...given 18 in the States is actually the minority AOC there.
I actually think it's Hollywood's (unintentional) fault for the myth that AOC is 18 in the US. Because a ton of our popular media comes from there, and California is one of those 18 states, so complying with the state law vastly over-represented the situation.