The all important age of adulthood in the united states and many other countries. When you reach this age, you may, among other things:

Legally sign an agreement or petition.
Own a company.
Smoke Cigarettes.
Vote.
View Porn.
Be in Porn.
Own a car.
Serve in the military.
Move away from your parents (Without a legal agreement.)
Run for some public offices (but not the presidency, for which you must be 35.)