Why are you an adult at 18?

hu wazzit dat stated dat once u become 18 u are an adult? scientists? government? or its a religion ting?

It's a government thing, in other countries, the legal age for being an adult is older or younger.

Same with marrying cousins.....

18 is an arbitrary number. Obviously different people mature at different rates. I've met 18 year olds who couldn't hold down a job, do their laundry, keep up with bills, etc. Luckily for them their parents didn't push them out on their own too soon. I've also met minors who due to circumstances were forced to be on their own and had no problems.

Legally it is necessary to draw the line somewhere. Until 18 your parents are responsible for you and your actions. After 18 you are responsible for yourself and your actions.