I'll be the first to say, different countries have different rules because of different situations. With that in mind, 16 actually is the age of consent in multiple US states. AFAIK, there's rules attached to it (like you can't be the 16 year old's teacher, boss, ect), but its a myth that the age of consent is 18 everywhere here. I still don't think anyone 20 and over should be sleeping with 16 year old, but that's mostly because 20 year olds should be at the point in their lives where their interests should be radically different from a 16 year old.
Typically, 20 year olds should be worried about working or going to higher education so they can pay bills. A 16 year old is still in highschool talking about fortnite and BTS. There's no reason for a 20 year old to be fucking a 16 year old, but I'd say that's my American mindset talking.