Plugin not picked up in EE3

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');

class Age_gate
{
  public static $name         = 'Age Gate';
  public static $version      = '1.0';
  public static $author       = 'Me';
  public static $author_url   = 'http://example.com/';
  public static $description  = 'Makes people verify their age.';
  public static $typography   = FALSE;


    public $return_data = "";

    public function __construct()
    {
        $this->return_data = "Hello World";
    }
}

/* End of file pi.plugin_name.php */
/* Location: ./system/user/addons/age_gate/pi.age_gate.php */