Adult age varies from country to country. In the UK you are not adult until the age of 18.
This means that without parental consent you cannot marry.
You cannot sign a contract; buy a house; obtain credit; join the Armed Services, or the Police (Except as a cadet!). Most of all, you cannot vote!
You can be on this site, of course, at thirteen.
Legally, until you reach 18, you are a child. I know it sounds ridiculous, but it's true!