Many times, I hear the phrase "grow up kid" and find it really ironic when it comes from the mouth of a child, especially over the internet. For example, when I intentionally troll kids on a Minecraft server, they always cry about my "your mom" jokes and "no u" replies. One of the benefits of being an adult is that you are smarter, obviously, and it usually is advantageous when dealing with funny kids.
An adult is a person who has reached maturity in bodily growth, mental development, or age--in other words, who is "grown up." Physiologically, the person must have achieved full growth and strength, and the age may vary with sex, race, and culture. Laws usually set the age at twenty-one and the effect is that the person attains to certain rights, such as voting, legal right to make contracts, etc. The age at which a person is considered adult has varied greatly among different people. In the United States, where the definition of an adult is made by the states, there has been much agitation since World War II to lower the age to 18 and some states have done so.