You can implement these two things using ASCII code & parseint method. Code is provided below :

//called when key is pressed in textbox
    $("#quantity").keypress(function (e)  
    { 
    $("#errmsg").hide();
      //if the letter is not digit then display error and don't type anything
      if( e.which!=8 && e.which!=0 && (e.which<48 || e.which>57))
      {
        //display error message
        $("#errmsg").html("&nbsp;&nbsp;Digits Only").show(); 
       return false;
      } 
      });
      $('#quantity').blur(function(a) {
      if((parseInt($("#quantity").val(), 10)<18)) 
      {
      $("#errmsg").html("&nbsp;&nbsp;Age must be greater than 18").show(); 

      }
      if((parseInt($("#quantity").val(), 10)>35)) 
      {
      $("#errmsg").html("&nbsp;&nbsp;Age must be less than 35").show(); 
      }
     });