the onclick method is giving me an Uncaught TypeError: Cannot set property 'onclick' of null error

Question

need help why is it throwing me this error:

Uncaught TypeError: Cannot set property 'onclick' of null

Here is the code:

 var age = document.getElementById( "number".value);
  document.getElementById('button').onclick = function()
  {
   if (age > 1 && age <=13)
  {
  alert('you are a child'); 
  } 
  else if (age >13 && age <=19)
  {
  alert('you are a teenager')
  }
  else if (age >20 && age <=65)
  {
  alert ('you are a young adult but still considered YOUNG :D');
  }
  else if (age >65 && age <=80)
  {
  alert( 'you are  a DOM (Dirty Old Man) hahaha'); 
  }
  else if (age >80 && age <=95)
  {
  alert('you're a pensioner');
  }
  else if (age >117 && age <=999)
  {
  alert('you are DEADZZZ');
  }
  };

Answer 1

getElementById will return null if there is no element found with that ID.