Edit in JSFiddle

function btn_prompt() {


var age = prompt("your age", "0");
if(age>=20){
	document.write("your adult");
}
else{
	document.write("your minor");
}
}


function btn_prompt2() {


  var age = prompt("your age", "0");
  if (age >= 20) {
     document.getElementById("prompt_text").innerHTML = "your adult";
     //$('#prompt_text').text("your adult");
  } else {
     document.getElementById("prompt_text").innerHTML = "your minor";
     //$('#prompt_text').text("your minor");
  }
}

<h1>
자바스크립트 맛보기
</h1>
<h2>
  age question
</h2>
<button onclick="btn_prompt()">
맛보기 실행
</button>

<button onclick="btn_prompt2()">
맛보기 실행2
</button>
<div id="prompt_text"></div>