According to the Alaska Bar Association, when an individual reaches the age of 18, he can marry, vote, buy tobacco products, engage in consensual sex, join the military, work full-time, apply for loans and credit and get piercings and tattoos. The National Conference of State Legislatures explains that rules vary from state-to-state, but most states consider an individual an adult at age 18.
As an adult, the individual has all of the rights and responsibilities as other adults, notes the National Conference of State Legislatures. If an adult signs a contract for indebtedness, the individual is held responsible for that debt. There are greater penalties and repercussions for an adult who commits a criminal offense than for a minor who commits the same offense.
The only activity prohibited at age 18 is the purchase and consumption of alcoholic beverages. Another activity legal in certain states, possession of marijuana, is generally prohibited before the age of 21, reports NBC News.
A mandated activity upon reaching adulthood is registration with the Selective Service within 30 days of a man's 18th birthday. The Selective Service System drafts men into the military, but the system has not been used to draft anyone since 1973, according to U.S. government expert Robert Longley for About.com.