I'm wondering what the laws are regarding residency and age of consent.
For no crime to be committed do both parties have to be over the age of consent in the state in which the sex act occurs, the states in which they each respectively live, or some combination of all three? I have been under the impression that only the laws of the state in which the sex act occurs are applicable, is this true?

If a person were to meet a person under the age of 18 for the first time in another state, the only laws that would apply would be that of the state they are both in at the time.

However, if a person takes a minor under the age of 18 across state lines, or entices a minor under the age of 18 to cross state lines, with the intent to engage in sexual activity, it is a federal crime, regardless of the age of consent in either state. The laws of the state which the non-minor person resides is irrelavent.

Any other sort of situation (e.g. two people who know each other are going to happen to be going on a trip to the same other state), would be a gray area that would need an attorney's advice on the specific situation.