Here is a patch to fix this. New version will be out soon to include this.
Open applications/birthdaygreeter/modules/admin/settings/settings.php
Find:
$form->add( new \IPS\Helpers\Form\NumberRange( 'abg_age_range', \IPS\Settings::i()->abg_age_range ? array( 'start' => reset( $ageRange ), 'end' => end( $ageRange ) ) : array( 'start' => 0, 'end' => 100 ), FALSE, array(), NULL, NULL, NULL, 'abg_age_range' ) );
Replace With:
$form->add( new \IPS\Helpers\Form\Custom( 'abg_age_range', \IPS\Settings::i()->abg_age_range ? array( 'start' => reset( $ageRange ), 'end' => end( $ageRange ) ) : array( 'start' => 0, 'end' => 100 ), FALSE, array( 'getHtml' => function( $element )
{
return "<input type='text' value='{$element->value['start']}' name='abg_age_range[start]' min='0' class='ipsField_short'> - <input type='text' value='{$element->value['end']}' name='abg_age_range[end]' min='0' class='ipsField_short'>";
} ), NULL, NULL, NULL, 'abg_age_range' ) );
Save.