Turning 18 years old is a gateway age, taking you across the border from childhood to adulthood. When you are a child, you long to be a grown-up, and it's true that lots of rights accrue to you. On the other hand, being an adult also comes with plenty of responsibilities.
TL;DR (Too Long; Didn't Read)
After you come of age, you get to vote and marry, if you want to. You also can enter into legal contracts, write a will and take control of your medical decisions. You can also do many things on your own that, when you were a minor, required parental consent, such as joining the military.
Coming of Age
You are said to come of age when you attain the age of majority. Although everyone gets the right to vote at the age of 18, thanks to the 26th Amendment to the Constitution, the age of majority for all other purposes is determined by each state. Most state laws put the age of majority at 18, but a few don't. You have to be 19 before you are considered an adult in Alabama and Nebraska, and 21 in Mississippi.
Rights You Get by Being 18 Years Old
The benefits of being 18 include most of the rights that adults enjoy, at least in those states with 18 as the age of majority. Suddenly, you don't have to ask your parents for permission to get a driver's license, get a tattoo or get elective surgery. You can move out of the family house just because you want to, get married, quit school and hitchhike around the galaxy.
Your legal rights expand. While you are a minor, you cannot enter into a binding contract. That means that if you do enter into a contract, the other person cannot sue if you don't follow its terms. Once you reach the age of majority, you can enter into a binding contract. That opens up the door to buying a car, getting a credit card and renting an apartment. You can also write a will and bring a lawsuit in your own name.
One benefit you do not get is freedom to drink alcohol. All states set the age limit for drinking at 21 years old, although a few allow for light drinking with family at an earlier age.
Downsides of Being 18 Years Old
Once you cross that magic threshold, you get the rights of an adult but also the concomitant responsibilities. If you pack up and leave the family home, your parents are not responsible for making sure you have enough money to take care of yourself. You want to rent an apartment or buy a car? It's up to you to pay for it. If you enter into a contract, you can be sued for failing to keep its terms.
Being an adult has both advantages and disadvantages. You'll likely learn something about both the benefits and the detriments when your 18th birthday rolls round.