With PHP, you can validate the form with something like:
$enteredAge = 17; //from the user
$minAge = 18;
$maxAge = 125; //debatable
if( !in_array( $enteredAge, range( $minAge, $maxAge) ) ){
//Incorrect age.
}
If you feel as though an array is too "expensive" for this type of thing:
$enteredAge = 17; //from the user
$minAge = 18;
$maxAge = 125; //debatable
if($enteredAge < $minAge || $enteredAge > $maxAge){
//Incorrect age.
}