This is a simple answer given to a child when they want to do what their parent is doing.

"Mummy, can I have some Coke?"
"No."
"But you always drink Coke! Why can't I?"
"Because I am an adult, and you are not."

It can also be said in the form "Because I am older than you" which is for use of an older sibling to a younger one, and can be accompanied by "Because I didn't get to when I was your age." If a parent didn't use this phrase they might say a simple "no". The word "no" says that no argument can be entered into, and this is they way it will always be, you will never be allowed to drink Coke simply because you cannot. "Because I'm an adult" means that one day, you will be able to.

The word "adult" is mostly a legal term, and means that they can drink, they can drive, they can buy a house and be judged in court as an adult. Of course, having a certain number attached to their name doesn't mean that one is emotionally mature enough to handle the responsibility of being an adult. People grow up at different rates.

A child shouldn't do many of the things that an adult does. A child shouldn't stay up late because they need their sleep more than an adult as a child is still developing. The argument can be made that adults shouldn't do many of the things adults do, staying up late isn't healthy for everyone, drinking liberal amounts of any soft drink, eating pizza for breakfast, none are too healthy. But see, a child is too young and too immature to understand the consequences of their actions. An "adult" does understand that eating pizza for breakfast is not healthy, they understand the consequences of their actions.

Of course, the adult should enter into some explanation before using this phrase. Otherwise it is akin to a simple "because", which is probably the most annoying answer anyone can give to anything. An adult shouldn't just tell the child no because it is within their power to do so. If there is no reason for their "no", then the adult is just abusing their power.

A parent's role as a parent is to take care of their children and make these sorts of decisions for them. Once the child has grown up the parent should and will let their kid make more and more decisions. They'll be reminded less often to get to bed by a certain time, they'll be allowed to drink what they want when they think they need it and slowly the child will have more responsibility over their own life. The parent will use the phrase "Because I'm and adult" less and less.

And once the child has grown up they will use the phrase "Because I am an adult".