Age 18 in the United States is considered the "age of majority" and is a milestone.
Age 18 has been the age of majority since the 26th Amendment, signed into law by President Nixon in July 1971. Historically the age of majority was 21, but when Franklin D. Roosevelt lowered the military draft age to 18 during World War II, there was a push to align the voting age with the military draft age.
So what changes when you turn 18? The State Bar of California has a nice guide for teenagers to help better understand how their rights and responsibilities change at age 18.
Here's a summary of these rights and responsibilities, excerpted from "When You Turn 18: A Survival Guide for Teenagers." (pdf link)
Enter into binding contracts - for example, leases, opening bank accounts and applying for loans (of course, to obtain the loan, you may still need a co-signer).
Buy or sell property, such as real estate and stock.
Marry without written consent of parents or guardian.
Sue or be sued.
Compromise, settle or arbitrate a claim.
Make or revoke a will.
Inherit property outright.
Vote in federal, state and local elections.
Consent to all types of medical treatment.
Join the military without parental consent. Male U.S. citizens or immigrants living in the U.S. generally must register with the Selective Service System within 30 days of turning 18. Women are exempt.
Get a job without a special work permit.
Serve jury duty.
Be subject to more serious consequences for breaking the law.
Prior to June 9, 2016, you could buy tobacco products; the law was changed, however, and now you have to wait until you are age 21.
Learn more at www.calbar.ca.gov.
For detailed guides in pdf format, visit www.calbar.ca.gov/Public/Free-Legal-Information/Legal-Guides.