Edit in JSFiddle

$(document).ready(function () {
  //on keypressed in textbox
  $("#age").keypress(function (e) {
     //if not numeric, then it don't let you type
     if (e.which != 8 && e.which != 0 && (e.which < 48 || e.which > 57)) {
        //display error message
        $("#errmsg").html("Digits Only").show().fadeOut("slow");
               return false;
    }
   });
});

Number : <input type="text" name="age" id="age" />&nbsp;<span id="errmsg"></span>

#errmsg
{
color: red;
}