Minimum Age Check on a Gravity Forms Date of Birth

$wpide_gf_min_age = 18;

add_filter( 'gform_field_validation', function ( $result, $value, $form, $field ) use ( $wpide_gf_min_age ) {
    if ( 'date' !== $field->type || ! $result['is_valid'] || false === strpos( ' ' . $field->cssClass . ' ', ' date-of-birth ' ) ) {
        return $result;
    }

    if ( is_array( $value ) ) {
        $value = implode( '/', array_values( $value ) );
    }

    $format = ! empty( $field->dateFormat ) ? $field->dateFormat : 'mdy';
    $parts  = preg_split( '/[\/\-.]/', (string) $value );

    if ( 3 !== count( $parts ) ) {
        return $result;
    }

    $map = array( 'mdy' => array( 2, 0, 1 ), 'dmy' => array( 2, 1, 0 ), 'dmy_dash' => array( 2, 1, 0 ), 'dmy_dot' => array( 2, 1, 0 ), 'ymd_slash' => array( 0, 1, 2 ), 'ymd_dash' => array( 0, 1, 2 ), 'ymd_dot' => array( 0, 1, 2 ) );
    $idx = isset( $map[ $format ] ) ? $map[ $format ] : $map['mdy'];

    $birth = sprintf( '%04d-%02d-%02d', (int) $parts[ $idx[0] ], (int) $parts[ $idx[1] ], (int) $parts[ $idx[2] ] );
    $limit = wp_date( 'Y-m-d', strtotime( '-' . $wpide_gf_min_age . ' years' ) );

    if ( $birth > $limit ) {
        $result['is_valid'] = false;
        $result['message']  = sprintf( 'You must be at least %d years old.', $wpide_gf_min_age );
    }

    return $result;
}, 10, 4 );