function clearField(field) {
     if (field.value == field.defaultValue) field.value = "";
}

function checkField(field) {
     if (field.name == "age") {
          var age = field.value;
          // if the age field is not empty && is not numeric then
          if (age != "" && isNaN(age)) {
               alert("Your age should be a number only. Thanks");
               field.value = field.defaultValue;
          }
          else if (age != "" && (age < 18 || age > 21) ) {
               alert("Sorry, you need to be between 18 & 21");
               field.value = field.defaultValue;
           }
     }

     if (field.value == "") field.value = field.defaultValue;
}

</SCRIPT>

<FORM>
     <INPUT TYPE="text" NAME="name" VALUE="Name"
       onFocus="clearField(this);" onBlur="checkField(this);">
     <INPUT TYPE="text" NAME="email" VALUE="E-mail Address"
       onFocus="clearField(this);" onBlur="checkField(this);">
     <INPUT TYPE="text" NAME="age" VALUE="Your Age"
       onFocus="clearField(this);" onBlur="checkField(this);">
</FORM>