America does not have a national age of consent; each state sets their own. It’s 16 in 30 states, 17 in nine states, and 18 in 11 states.
Yes, the age of consent is 16 in the majority of the USA.
Several of the states which set it at 17 or 18 also have close in age exemptions (AKA Romeo and Juliet laws) which cover the case that OP raises. In such cases, it is legal to have sex with somebody slightly below the age of consent as long as both people are sufficiently close in age; some states, such as Texas, also require them to have been dating since both were under the state age of consent. This means that, in most of America, it is perfectly legal for a 19-year-old to have sex with a 16-year-old.
The misunderstanding most likely comes from the fact that the majority of entertainment media is made in New York and California, where the age of consent is 18 with no exceptions, and pop culture writers act like this is the case everywhere.
Also I should clarify that bringing a minor across state limes to take advantage of a lower age of consent is a felony, for obvious reasons.
That’s my guess. I had the same experience tge first time I followed a hashtag, and I’ve seen it happen to at least one other person.