Javascript if/else statement that makes a user enter a value

How would you make an if else statement loop back to the beginning to get user information?

This is the code I got so far:

var age = prompt("Please enter your age");

if(age == 21 ){
alert("Happy 21st Birthday!");
}else if (age > 21 ){
alert("You are old");
}else{ 
alert("Please enter an age");
}

I'm trying to make it go back to the beginning to make the user enter information.