Age of consent
The age of consent is the age in which it is legal to engage in sexual activity in a given jurisdiction. Most nations have an age of consent but in some cases such as the United States the age of consent laws are set by sub-national jurisdictions such as states.
Many jurisdictions have close-in-age exemptions in which sex is not illegal between people of similar age even if it otherwise would be. In some countries, these are called Romeo and Juliette laws.