The age of majority in Missouri is 18. On that birthday you become legally competent to sign contracts, make your own medical decisions, sue and be sued in your own name, and manage your own affairs without a parent’s involvement. A handful of rights and obligations, though, arrive earlier or later than 18, and knowing which is which prevents some common surprises.
What Changes Legally at 18
Missouri Revised Statutes Section 431.055 sets 18 as the age at which a person becomes competent to contract.1Missouri Revisor of Statutes. Missouri Revised Statutes 431.055 – Persons Competent to Contract When Eighteen Years of Age Before that birthday, most agreements a minor signs are voidable, meaning the minor can walk away from them. After 18, a contract you sign is binding and enforceable against you.
Turning 18 also ends the default legal relationship between parent and child. Parents no longer have authority to make decisions on your behalf, and you no longer need their permission to act. You can file lawsuits in your own name without a guardian ad litem, own real property outright, and handle your own finances. In criminal court, anyone 18 or older faces adult prosecution and sentencing.
Rights That Arrive at 18
The most visible new rights at 18 are voting, serving on a jury, and entering binding contracts for things like apartment leases and car loans.1Missouri Revisor of Statutes. Missouri Revised Statutes 431.055 – Persons Competent to Contract When Eighteen Years of Age
You also gain full authority over your own medical care. You consent to treatment, choose your providers, and access your own records without parental involvement. Missouri’s advance directive registry statute, Section 459.250, defines “adult” as someone 18 or older and lets adults sign living wills, do-not-resuscitate orders, and healthcare powers of attorney.2Missouri Revisor of Statutes. Missouri Revised Statutes 459.250 – Registry Established, Definitions
At 18 you can register as an organ donor, and that decision is legally yours alone. Under the Uniform Anatomical Gift Act as adopted in Missouri, an anatomical gift made by a person 18 or older is irrevocable after death and does not require family consent.3U.S. Department of Health and Human Services. Recommendations 19-28 In practice, some organ procurement organizations still defer to objecting families, but the law is on the donor’s side.
Federal Obligations That Start at 18
Selective Service Registration
Almost all male U.S. citizens and male immigrants must register with the Selective Service System within 30 days of turning 18.4Selective Service System. Who Needs to Register The obligation continues through age 25. Failing to register is a federal offense punishable by a fine up to $10,000 or imprisonment up to five years, though criminal prosecutions are rare.5Office of the Law Revision Counsel. 50 USC 3811 – Offenses and Penalties The consequence that bites most people is loss of eligibility for federal student financial aid, federal job training programs, and federal civilian employment.
Social Security Child Benefits
If you receive Social Security benefits as the child of a retired, disabled, or deceased worker, those benefits generally end the month before you turn 18. If you’re a full-time secondary school student at 18, benefits can continue until you finish high school or turn 19, whichever comes first.6Social Security Administration. Frequently Asked Questions for Students There is no extension for college. Benefits also stop if you marry, drop below full-time attendance, or leave school.
Ages in Missouri That Aren’t 18
Custodial Accounts Transfer at 21
If a parent or grandparent set up a custodial account for you under Missouri’s Uniform Transfers to Minors Act, you may not control it at 18. Under Section 404.051, custodial property created by a donor’s gift, will, or trust transfers to the beneficiary at age 21. Property transferred by someone other than a donor is the exception; there, the minor can request the funds at 18.7Missouri Revisor of Statutes. Missouri Revised Statutes 404.051 Families are often caught off guard when an 18-year-old expects to access college savings and learns the custodian legally controls the account for three more years.
Credit Cards
Federal law adds another wrinkle. Under the Credit CARD Act, a card issuer generally cannot open an account for anyone under 21 unless the applicant shows an independent ability to make the minimum payments.8Federal Deposit Insurance Corporation. ECOA – Understanding Age-Based Discrimination in Credit Card Lending Turning 18 makes you old enough to sign a credit agreement under Missouri contract law, but most 18-to-20-year-olds either need verifiable income or must be added as an authorized user on someone else’s account.
The Kiddie Tax
Investment income can still be taxed at your parents’ rate after your 18th birthday. Under the IRS “kiddie tax” rules, a child’s unearned income can be taxed at the parents’ marginal rate if the child is under 18, is 18 and doesn’t earn more than half of their own support, or is a full-time student under 24 who doesn’t earn more than half of their own support.9Internal Revenue Service. Topic No. 553, Tax on a Child’s Investment and Other Unearned Income (Kiddie Tax)
Handguns at 21
At 18 you can buy long guns like rifles and shotguns from a federally licensed dealer. You must be 21 to buy a handgun or handgun ammunition from an FFL dealer under federal law.10Bureau of Alcohol, Tobacco, Firearms and Explosives. Minimum Age for Gun Sales and Transfers Private sales follow different rules, and Missouri does not add state-level age restrictions beyond the federal minimums.
Work at 16
Missouri’s child labor laws center on 16, not 18. Section 294.011 defines “child” as someone under 16, and Section 294.040 prohibits employing anyone under 16 in hazardous work involving power-driven machinery, motor vehicles, mines, blast furnaces, toxic chemicals, and other dangerous settings.11Missouri Revisor of Statutes. Missouri Revised Statutes 294.040 At 16 most of those restrictions lift, and at 18 the remaining state and federal work-hour limits for minors fall away entirely.
When Child Support Actually Ends
Turning 18 doesn’t automatically cut off child support in every case. Under Section 452.340, a parent’s support obligation generally ends when the child reaches 18, but Missouri law extends the deadline in two important situations.12Missouri Revisor of Statutes. Missouri Revised Statutes 452.340 – Child Support
First, if the child is still enrolled in and attending a secondary school program at 18, support continues until the child finishes or turns 21, whichever comes first. Second, if the child enrolls in a vocational or higher education program no later than October 1 after graduating high school and carries at least 12 credit hours per semester with passing grades, support can continue until the child finishes that program or reaches 21. A court can also extend support past 18 indefinitely if the child has a physical or mental incapacity that prevents self-support.
Support ends early if the child marries, enters active military service, or becomes self-supporting with the custodial parent’s consent. Those triggers apply regardless of age.
Ways Minors Reach Adult Status Early
Military Enlistment
Federal law lets 17-year-olds enlist in the armed forces with parental consent. A minor who enters active military duty is treated as an adult under Missouri child support law, and the parents’ support obligation ends at that point.12Missouri Revisor of Statutes. Missouri Revised Statutes 452.340 – Child Support
Emancipation
Missouri does not have a single, comprehensive emancipation statute the way some states do. Courts can grant emancipation case by case, and the general expectation is that the minor must be at least 16, a Missouri resident, and financially self-supporting without public assistance. Because there is no clearly codified process, emancipation in Missouri typically requires an attorney petitioning the court directly, and outcomes depend heavily on the judge’s assessment of the minor’s maturity and circumstances.
Minor Consent for Specific Medical Care
Even without full emancipation, Missouri law lets minors consent to their own care in specific areas. Under Sections 431.061 and 431.062, minors can consent to treatment for sexually transmitted infections and for drug or alcohol treatment without parental involvement.13Missouri Department of Health and Senior Services. Missouri Minor Consent Laws Separate statutes address a minor’s admission to inpatient mental health services.
Marriage Is No Longer a Path
Missouri overhauled its marriage laws effective August 28, 2024. Under the previous rules, 16- and 17-year-olds could marry with parental consent. That is no longer the case. No marriage license may be issued in Missouri for anyone under 18.14Missouri Senate. SB767 – Modifies Provisions Relating to Marriage Older guides listing 16 as the minimum are outdated. A minor who was already legally married before the law changed is still considered an adult for purposes of consenting to medical or surgical treatment under Section 431.065.