Citizens at age 18 are legally considered adults.
In the United States adulthood is reached at 18 along with that certain responsibilities and privileges arise.
1.) Males must register for selective service
http://www.sss.gov/QUICK.HTM
2.) All citizens age 18 and older have the right to vote.
26th Amendment to the US constitution.
3.) Parents are no longer required to care for their children past age 18. This means citizens age 18 and over are responsible for and/or have the right to engage in: Contracts, Marriage, Divorce, Employment, Jury Duty, Torts, and Transportation.
http://education.occourts.org/lfyrights.asp