16 will get you 20 is actually a fallacy, based on the rules for pornography in the United States. While it is true that a minor (under 18) may not appear nude in any form of media, there is no federal law mandating a legal age of consent. The age of consent is defined by the state government, not the federal one.
The most common age of consent in the United States is 16; however some states have 17 or 18 as the legal age of consent. It is however a federal crime to cross state lines with a minor in order to have sex legally; you cannot take your 16 year old girlfriend from NY to New Hampshire just to have sex.
Most states also have a close in age exemption of around four years, where teens aged 14+ may have intercourse with others within four years of their own age. Numbers vary state to state, some being 16+, and some with 3 or 5 years; and not every state has such rules.
Dude 1: Man, your gf is total jailbait; you better watch it or you'll wind up in the pen. 16 will get you 20!
Dude 2: We live in Indiana, dumbass. Legal age of consent is 16.