I think that the problem is that you are trying to echo undefined variables as the values in your input boxes and this is why you are getting the errors. The reason these are echoed this way is because without them, if someone enters data, and your code does not like what is entered and displays an error message, all the inputs will go away.
A common way to prevent this error is to set all of those variables to empty in the beginning. Try putting the below code at the top of the file with the rest of your variables and see what that does. I think it will fix your problem, although I could be wrong.
$strForename = $strAge = $strEmail = '';
Just a cursory glance and I notice your first issue is that your setting variables with unknown or empty values which are undefined
try doing this
$strSuggestion = $_POST["suggestion"];
then just check if the variables are null before doing other checks
$strSuggestion = isset($_POST["suggestion"]) ? $_POST["suggestion"] : null;
this should resolve your undefined issues.
$strError .= "<li>Your error message</li>\n";