Isabella is a fourteen-year-old girl living in Houston, Texas. Her family is planning a special party for her fifteenth birthday, called a quinceañera. After this, Isabella will be considered a young woman by her community. In years past, and even now in many places, young people were considered adults as soon as they completed a rite of passage, a special ceremony to mark a change of status. In the United States today, though, it is difficult to know when adulthood really begins and childhood ends. The word “adult” is quite ambiguous, because the assignment of adult status varies across domains.

Most states have laws allowing marriage at the age of 18. But in some states, if a parent gives permission, minors can get married as young as 16. In rare cases, minors can get married even younger with a court’s approval.

The legal age for voting or joining the military is 18, but those under the age of 21 are not considered mature enough to buy alcohol or, in most states, to rent a car. And clients aged 21–25 are charged a large extra daily insurance fee by most car rental companies.

The passage of the Affordable Care Act enabled parents to keep their children on their health insurance plans until they turn 26. This change acknowledged that people under 26 years old are less likely to have a stable job and lifestyle, typical markers of adulthood.

Due to their age, workers under 20 years old do not have to be paid the minimum wage during their first 90 days of work. This guideline under federal law is called the “youth minimum wage.” Therefore, you are not an adult until age 20 according to the U.S. Department of Labor.

In many states, children as young as 8 years old can be charged as adults for committing certain crimes. In fact, almost 3,000 people under the age of 18, some as young as 13, have been sentenced to life in prison without parole. This means that children can be sentenced to spend the rest of their lives in jail because of something they did in middle school. As of 2016, on any given day, 10,000 people under the age of 18 are in the same prison system as adults.

Look at the student next to you. Do you consider this person an adult?