How to restrict an age in this code ?

See more:

Javascript

var x=prompt("enter age here", "10");
if(x>60)
{
document.write("older");
}
else if(x>40)
{
document.write('Medium age');
}
else if(x>18)
{
document.write('young');
}
else if (x>0)
{
document.write('child');
}
else {
alert("invalid input");
}

What I have tried:

if i am entering a number more than 60 even any number like 200,300,2000,2000000, getting answer is 'older'. Can anybody tell me how to validate it i mean age should not be more than 120 years.